Cleanup issue template.

1. Move all the comment text to the bottom of the template.  This allows
one to fill in the relevant fields without having to look for them
through a wall of text.

2. Reword the comments in a slightly more compact/concise manner (I
believe we can maintain a friendly tone without repeating "please" 9
times in 15 sentences).

Author: anntzer

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,27 +1,5 @@
-<!--Thank you so much for your PR! To help us review, fill out the form
-to the best of your ability.  Please make use of the development guide at
-https://matplotlib.org/devdocs/devel/index.html
-
-For help with git and github workflow, please see https://matplotlib.org/devel/gitwash/development_workflow.html
-
-Please do not create the PR out of master, but out of a separate branch. -->
-
-<!--Provide a general summary of your changes in the title above, for
-example "Raises ValueError on Non-Numeric Input to set_xlim".  Please avoid
-non-descriptive titles such as "Addresses issue #8576".-->
-
-<!--If you are contributing fixes to docstrings, please pay attention to
-http://matplotlib.org/devel/documenting_mpl.html#formatting.  In particular,
-note the difference between using single backquotes, double backquotes, and
-asterisks in the markup.-->
-
 ## PR Summary
 
-<!--Please provide at least 1-2 sentences describing the pull request in
-detail.  Why is this change required?  What problem does it solve?-->
-
-<!--If it fixes an open issue, please link to the issue here.-->
-
 ## PR Checklist
 
 - [ ] Has Pytest style unit tests
@@ -31,8 +9,33 @@ detail.  Why is this change required?  What problem does it solve?-->
 - [ ] Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
 - [ ] Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way
 
-<!--We understand that PRs can sometimes be overwhelming, especially as the
-reviews start coming in.  Please let us know if the reviews are unclear or the
-recommended next step seems overly demanding , or if you would like help in
-addressing a reviewer's comments.  And please ping us if you've been waiting
-too long to hear back on your PR.-->
+<!--
+Thank you so much for your PR!  To help us review your contribution, please
+consider the following points:
+
+- A development guide is available at https://matplotlib.org/devdocs/devel/index.html.
+
+- Help with git and github is available at
+  https://matplotlib.org/devel/gitwash/development_workflow.html.
+
+- Do not create the PR out of master, but out of a separate branch.
+
+- The PR title should summarize the changes, for example "Raise ValueError on
+  non-numeric input to set_xlim".  Avoid non-descriptive titles such as
+  "Addresses issue #8576".
+
+- The summary should provide at least 1-2 sentences describing the pull request
+  in detail (Why is this change required?  What problem does it solve?) and
+  link to any relevant issues.
+
+- If you are contributing fixes to docstrings, please pay attention to
+  http://matplotlib.org/devel/documenting_mpl.html#formatting.  In particular,
+  note the difference between using single backquotes, double backquotes, and
+  asterisks in the markup.
+
+We understand that PRs can sometimes be overwhelming, especially as the
+reviews start coming in.  Please let us know if the reviews are unclear or
+the recommended next step seems overly demanding, if you would like help in
+addressing a reviewer's comments, or if you have been waiting too long to hear
+back on your PR.
+-->