core: more exact test on the procfs special string "(deleted)"

In other parts of systemd, the code checks for " (deleted)",
but in one instance, it did not (yet). Make it do the same.
doc: quote consistently in autoconf code

AS_HELP_STRING has been observed to expand such that the surround
function complains; play it safe and consistenly quote the example
code throughout.
doc: detail what "UID" is
doc: update punctuation

Resolve spotted issues related to missing or extraneous commas, dashes.
doc: resolve missing/extraneous words or inappropriate forms

Issues fixed:
* missing words required by grammar
* duplicated or extraneous words
* inappropriate forms (e.g. singular/plural), and declinations
* orthographic misspellings
doc: choose different words to improve clarity

I suggest the following changes to improve the way the text reads
("flows").
doc: properly use XML entities
doc: balance C indirections in function prototypes

Shift the asterisks in the documentation's prototypes such that they
are consistent among each other. Use the right side to match source code.
doc: reword "shared per-thread" wording

Either it is shared across threads, or it is per-thread: decide.

Reading the source code, I see a thread_local identifier, so that's
that. But that does not yet preclude that a program may pass around
the pointer returned from the function among its own threads.

Do a best effort at saying so.
doc: utilize the DocBook markup for some literals