Documentation/filesystems/proc.rst: copy-editing cleanup

Clean up Documentation/filesystems/proc.rst.

This is basically fixing lots of spelling, grammar, punctuation,
typos, spacing, consistency, section numbering, and headings.
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/a5f126e6-d67a-154a-1c87-d8f07542a21c@infradead.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>