pandoc: improve desc, add 2 examples (#1072)

The list of supported formats needs to be done this way until https://github.com/jgm/pandoc/issues/2685 is resolved.

Also expand the --to option, to avoid ambiguity (per PR review)

pages/common/pandoc.md

@@ -1,7 +1,15 @@
 # pandoc
 
-> General markup converter.
+> Convert documents between various formats.
 
 - Convert file to pdf (the output format is automatically determined from the output file's extension):
 
 `pandoc {{input.md}} -o {{output.pdf}}`
+
+- Convert a file to a specific output format (useful for when the extension alone is ambiguous):
+
+`pandoc {{input.docx}} --to {{markdown_github}} -o {{output.md}}`
+
+- List all supported input and output formats:
+
+`pandoc --help | head`