runtime(doc): document pandoc compiler and enable configuring arguments

closes: #14550

Signed-off-by: Konfekt <Konfekt@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/compiler/pandoc.vim b/runtime/compiler/pandoc.vim
index 1b171a1..6c15193 100644
--- a/runtime/compiler/pandoc.vim
+++ b/runtime/compiler/pandoc.vim
@@ -46,6 +46,7 @@
       \ '\ --metadata\ title=%:t:r:S' .
       \ '\ --metadata\ lang=' . matchstr(&spelllang, '^\a\a') .
       \ '\ --from=' . s:PandocFiletype(&filetype) .
+      \ '\ ' . escape(get(b:, 'pandoc_compiler_args', get(g:, 'pandoc_compiler_args', '')), ' ') .
       \ '\ --output\ %:r:S.$*\ %:S'
 
 CompilerSet errorformat="%f",\ line\ %l:\ %m