doc/cephfs: Small improvements in fscrypt.rst

doc/cephfs: Small improvements in fscrypt.rst

Fix Sphinx warnings about section title underline lengths.
Use title case in section titles.

Change Unicode quotation marks to ASCII.

Use ordered list for lines that were supposedly intended to be a list.

Use double backticks for literals.
Use image caption formatting.

Remove unnecessary comma and other small language improvements.

Capitalize MDS, OSD, etc.

Signed-off-by: Ville Ojamo <14869000+bluikko@users.noreply.github.com>