Minor fixes for upcoming version 13

Hello hackers,

Please consider fixing minor defects in the source code and documentation.
1. a missing dot in guc.c
"SELECT name, short_desc FROM pg_settings WHERE short_desc NOT LIKE
'%.'" finds only this one instance.
2. inrange -> in_range
the former spelling is unique
3. sigature -> signature
4. "start timeline %u not found history of timeline %u" -> "start
timeline %u not found in history of timeline %u"
it seems that a preposition is missing
5. <type>BackgroundWorker *worker</type> -> <parameter>BackgroundWorker
*worker</parameter>
incorrect Docbook tag
6. "unhandled previous state in xqs" -> "unhandled previous state at end
quote"?
"xqs" is rather cryptic for a user-visible message

I'm not sure about 6, so the attached patch covers only 1-5.

Best regards,
Alexander