Clarify and expand messages mentioning "loss of session data"

Clarify and expand messages mentioning "loss of session data"

Saying "loss of session data" in error messages is meaningless to the
user. It basically means "you might have been logged out", and is
sometimes normal (as login sessions are not stored forever).

MediaWiki gives this error when it isn't sure if it's still the same
user who started the edit, instead of just re-creating the session and
saving the page, which could potentially expose user's IP address.

Expanded 'session_fail_preview' message with this information, changed
which parts of it we bold, and updated three other messages for
consistency. Updated their documentation to refer for each other, too
(https://translatewiki.net/wiki/Template:Identical/Loss_of_session_data).

Bug: T125334
Change-Id: Ia99f39fac506855804ce5c40b0df3381d2782e48