Document that GTimeVal is subject to the year 2038 problem on 32-bit systems

It might not be immediately obvious that this is the case. Let's record
it in the description of `GTimeVal` itself and also in
`g_time_val_from_iso8601`.

We also drop an incorrect statement in the documentation for
`g_time_val_from_iso8601` stating that years up to 3000 were supported;
this is also not true for the same reason.

Related: #1509
9 jobs for wip/gtimeval-document-year-2038-problem in 7 minutes and 47 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #97375
cross-android_api21_arm64

00:01:12

passed #97376
cross-android_api28_arm64

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passed #97377
cross-mingw64

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passed #97374
fedora-autotools-x86_64

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passed #97373
fedora-x86_64

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manual #97380
freebsd-11 allowed to fail manual
freebsd-11-x86_64
passed #97378
win32
msys2-mingw32

00:06:24

passed #97379
win32
vs2017-x64

00:05:42

 
  Coverage
passed #97381
coverage

00:00:56

73.5%