Documentation/proc.txt: Add 2 missing fields for /proc/<pid>/status

It was found that two of the fields in the /proc/<pid>/status file were
missing - CapAmb & Speculation_Store_Bypass. They are now added to the
proc.txt documentation file.

v2: Update the example as well.

Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>