Help: Update FileAPI codemodel version to 2.1, add release notes

This fixes the documentation which should have been fixed in 9f6d40ee.

Help/manual/cmake-file-api.7.rst

@@ -425,7 +425,7 @@ Version 1 does not exist to avoid confusion with that from
 
   {
     "kind": "codemodel",
-    "version": { "major": 2, "minor": 0 },
+    "version": { "major": 2, "minor": 1 },
     "paths": {
       "source": "/path/to/top-level-source-dir",
       "build": "/path/to/top-level-build-dir"
@@ -915,6 +915,8 @@ with members:
       unsigned integer 0-based index into the ``backtraceGraph`` member's
       ``nodes`` array.
 
+    This field was added in codemodel version 2.1.
+
   ``defines``
     Optional member that is present when there are preprocessor definitions.
     The value is a JSON array with an entry for each definition.  Each

Help/release/3.18.rst

@@ -312,3 +312,9 @@ Other Changes
   network communication via ``http(s)``, such as :command:`file(DOWNLOAD)`,
   :command:`file(UPLOAD)`, and :command:`ctest_submit`.
   The precompiled binaries provided on ``cmake.org`` now support HTTP/2.
+
+* The :manual:`cmake-file-api(7)` "codemodel" version 2 ``version`` field has
+  been updated to 2.1.
+
+* The :manual:`cmake-file-api(7)` "codemodel" version 2 "target" object gained
+  a new ``precompileHeaders`` field in the ``compileGroups`` objects.