Cleanup: move public doc-strings into headers for 'blenkernel'

- Added space below non doc-string comments to make it clear
  these aren't comments for the symbols directly below them.
- Use doxy sections for some headers.
- Minor improvements to doc-strings.

Ref T92709

source/blender/python/intern/bpy_interface.c

@@ -895,8 +895,7 @@ bool BPY_string_is_keyword(const char *str)
   return false;
 }
 
-/* EVIL, define text.c functions here... */
-/* BKE_text.h */
+/* EVIL: define `text.c` functions here (declared in `BKE_text.h`). */
 int text_check_identifier_unicode(const uint ch)
 {
   return (ch < 255 && text_check_identifier((char)ch)) || Py_UNICODE_ISALNUM(ch);