Simplify headers and instructions

There is confusion with having sub headings below Description.
This commit removes teh Description header completely and promotes
sub headings to full headings.

Some of the notes under each heading have also been updated to
provide better clarity.

Author: adbridge

.github/pull_request_template.md

@@ -1,25 +1,38 @@
 <!--
 For more information on the requirements for pull requests, please see [the contributing guidelines](https://os.mbed.com/docs/mbed-os/latest/contributing/workflow.html).
+
+NOTE: Do not remove any of the template headings (even for optional sections) as this
+template is automatically parsed. 
 -->
 
-### Description 
-#### Summary of change <!-- Required -->
+### Summary of changes <!-- Required -->
+
+<!-- 
+    Please provide the following information: 
+
+    Description of the the change (what is this fixing / adding / removing?).
 
-<!-- Basic information about the change: what the change is for, why do we need it any implications -->
+    Why the change is needed (if this is fixing a reported issue please summarize what
+    the issue is and add the reference. E.g. Fixes #17119).
 
-#### Documentation <!-- Optional, but most likely you need it -->
+    Any implications for users taking this change.
+ -->
 
-<!-- Details of any document updates required, including links to PR against the docs repository -->
+### Documentation <!-- Required -->
+
+<!-- 
+    Please provide details of any document updates required, including links to any
+    related PRs against the docs repository.
+    If no document updates are required please specify 'None', this at least tells us
+    that this has been considered.
+-->
 
 ----------------------------------------------------------------------------------------------------------------
 ### Pull request type <!-- Required -->
 
 <!--
-    Required
     Please add only one X to one of the following types. Do not fill multiple types (split the pull request otherwise).
-    Please note this is not a GitHub task list, indenting the boxes or changing the format to add a '.' or '*' in front
-    of them would change the meaning incorrectly. The only changes to be made are to add a description text under the
-    description heading and to add a 'x' to the correct box.
+    Please note this is not a GitHub task list, indenting the boxes or changing the format to add a '.' or '*' in front of them would change the meaning incorrectly. 
 -->
     [] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
     [] Feature update (New feature / Functionality change / New API)
@@ -29,9 +42,7 @@ For more information on the requirements for pull requests, please see [the cont
 ### Test results <!-- Required -->
 
 <!--
-    Required
-    Provide all the information required, listing all the testing performed. For new targets please attach full test results
-    for all supported compilers.
+    Provide all the information required, listing all the testing performed. For new targets please attach full test results for all supported compilers.
 -->
     [] No Tests required for this change (E.g docs only update)
     [] Covered by existing mbed-os tests (Greentea or Unittest)
@@ -42,7 +53,6 @@ For more information on the requirements for pull requests, please see [the cont
 ### Reviewers <!-- Optional -->
 
 <!--
-    Optional
     Request additional reviewers with @username or @team
 -->