doc/cephfs: Fix Sphinx warnings and other minor improvements

Fix 2 Sphinx warnings about problematic syntax in troubleshooting.rst:
add empty lines between paragraphs.

In fs-volumes.rst:

Fix 1 Sphinx warning about problematic syntax:
Fix indentation and change to ordered list instead of manually
hardcoding item numbers.

Use privileged bash prompt consistently for privileged commands.

Use the "[...] run a command of the following form:" for CLI examples
with placeholders.

Fix accidental emphasis on a list item by adding empty lines around
sub-list.

Use title case in section titles and trim overly long underline.

Capitalize UID, GID, Unicode, Ceph in text.

Use double backticks consistently for command line switches, data, etc.

Start with capital case and end in full stop if full sentence.

Linkify mention of quiesce dev doc and add a label for it in
doc/dev/mds_internals/quiesce.rst.

Other very minor changes like typo, remove space at end of line.

Signed-off-by: Ville Ojamo <[email protected]>

Author: bluikko