Support `yaml_metadata_block` extension form commonmark, gfm.

This is a bit more limited than with markdown, as documented
in the manual:

- The YAML block must be the first thing in the input.
- The leaf notes are parsed in isolation from the rest of
  the document.  So, for example, you can't use reference
  links if the references are defined later in the document.

Closes #6537.

1 files changed, 16 insertions, 0 deletions

diff --git a/MANUAL.txt b/MANUAL.txt
index bc9e226e9..845d1dbba 100644
--- a/MANUAL.txt
+++ b/MANUAL.txt
@@ -4240,6 +4240,22 @@ will be interpreted as markdown. For example:
       \renewcommand{\section}[1]{\clearpage\oldsection{#1}}
       ```
 
+Note:  the `yaml_metadata_block` extension works with
+`commonmark` as well as `markdown` (and it is enabled by default
+in `gfm` and `commonmark_x`).  However, in these formats the
+following restrictions apply:
+
+- The YAML metadata block must occur at the beginning of the
+  document (and there can be only one).  If multiple files are
+  given as arguments to pandoc, only the first can be a YAML
+  metadata block.
+
+- The leaf nodes of the YAML structure are parsed in isolation from
+  each other and from the rest of the document.  So, for
+  example,  you can't use a reference link in these contexts
+  if the link definition is somewhere else in the document.
+
+
 ## Backslash escapes
 
 #### Extension: `all_symbols_escapable` ####