📚 Update #search documentation

* Expanded criteria docs:
  * explain sequence-set coercion
  * explain encoding of string, integer, and date args
  * add warning for raw string data
* Expanded charset docs
  * Link to IANA charset registry
  * Indicate defaults are "US-ASCII" or "UTF-8"
  * Demonstrate sending charset embedded inside criteria
* Move `#search` example above search criteria.  The list of search keys
  is long.  And to be more complete, it will need to get even longer.
  It's nice to have some examples near the top.
* Update `#search` example (`NEW` is deprecated)
  `NEW` and `RECENT` have been removed from `IMAP4rev2`, so I'm updating
  the example with a search key that isn't deprecated.

lib/net/imap.rb

@@ -1930,17 +1930,53 @@ def uid_expunge(uid_set)
     end
 
     # Sends a {SEARCH command [IMAP4rev1 §6.4.4]}[https://www.rfc-editor.org/rfc/rfc3501#section-6.4.4]
-    # to search the mailbox for messages that match the given searching
-    # criteria, and returns message sequence numbers.  +keys+ can either be a
-    # string holding the entire search string, or a single-dimension array of
-    # search keywords and arguments.
-    #
-    # Returns a SearchResult object.  SearchResult inherits from Array (for
+    # to search the mailbox for messages that match the given search +criteria+,
+    # and returns a SearchResult.  SearchResult inherits from Array (for
     # backward compatibility) but adds SearchResult#modseq when the +CONDSTORE+
     # capability has been enabled.
     #
+    # +criteria+ is one or more search keys and their arguments, which may be
+    # provided as an array or a string.
+    # See {"Search criteria"}[rdoc-ref:#search@Search+criteria], below.
+    #
+    # * When +criteria+ is an array, each member is a +SEARCH+ command argument:
+    #   * Any SequenceSet sends SequenceSet#valid_string.
+    #     +Range+, <tt>-1</tt>, and nested +Array+ elements are converted to
+    #     SequenceSet.
+    #   * Any +String+ is sent verbatim when it is a valid \IMAP atom,
+    #     and encoded as an \IMAP quoted or literal string otherwise.
+    #   * Any other +Integer+ (besides <tt>-1</tt>) will be sent as +#to_s+.
+    #   * +Date+ objects will be encoded as an \IMAP date (see ::encode_date).
+    #
+    # * When +criteria+ is a string, it will be sent directly to the server
+    #   <em>without any validation or encoding</em>.  *WARNING:* This is
+    #   vulnerable to injection attacks when external inputs are used.
+    #
+    # +charset+ is the name of the {registered character
+    # set}[https://www.iana.org/assignments/character-sets/character-sets.xhtml]
+    # used by strings in the search +criteria+.  When +charset+ isn't specified,
+    # either <tt>"US-ASCII"</tt> or <tt>"UTF-8"</tt> is assumed, depending on
+    # the server's capabilities.  +charset+ may be sent inside +criteria+
+    # instead of as a separate argument.
+    #
     # Related: #uid_search
     #
+    # ===== For example:
+    #
+    #   p imap.search(["SUBJECT", "hello", "NOT", "SEEN"])
+    #   #=> [1, 6, 7, 8]
+    #
+    # The following searches send the exact same command to the server:
+    #
+    #    # criteria array, charset arg
+    #    imap.search(%w[OR UNSEEN FLAGGED SUBJECT foo], "UTF-8")
+    #    # criteria string, charset arg
+    #    imap.search("OR UNSEEN FLAGGED SUBJECT foo", "UTF-8")
+    #    # criteria array contains charset arg
+    #    imap.search(%w[CHARSET UTF-8 OR UNSEEN FLAGGED SUBJECT foo])
+    #    # criteria string contains charset arg
+    #    imap.search("CHARSET UTF-8 OR UNSEEN FLAGGED SUBJECT foo")
+    #
     # ===== Search criteria
     #
     # For a full list of search criteria,
@@ -1982,18 +2018,13 @@ def uid_expunge(uid_set)
     #
     # TO <string>:: messages with <string> in their TO field.
     #
-    # ===== For example:
-    #
-    #   p imap.search(["SUBJECT", "hello", "NOT", "NEW"])
-    #   #=> [1, 6, 7, 8]
-    #
     # ===== Capabilities
     #
-    # If [CONDSTORE[https://www.rfc-editor.org/rfc/rfc7162.html]] is supported
+    # If CONDSTORE[https://www.rfc-editor.org/rfc/rfc7162.html] is supported
     # and enabled for the selected mailbox, a non-empty SearchResult will
     # include a +MODSEQ+ value.
     #   imap.select("mbox", condstore: true)
-    #   result = imap.search(["SUBJECT", "hi there", "not", "new")
+    #   result = imap.search(["SUBJECT", "hi there", "not", "new"])
     #   #=> Net::IMAP::SearchResult[1, 6, 7, 8, modseq: 5594]
     #   result.modseq # => 5594
     def search(keys, charset = nil)
@@ -2008,7 +2039,7 @@ def search(keys, charset = nil)
     # backward compatibility) but adds SearchResult#modseq when the +CONDSTORE+
     # capability has been enabled.
     #
-    # See #search for documentation of search criteria.
+    # See #search for documentation of parameters.
     def uid_search(keys, charset = nil)
       return search_internal("UID SEARCH", keys, charset)
     end