app, libgimp, pdb: improve gimp_vectors_stroke_get_length() docs.

The `precision` parameter in particular had no min/max, which meant we
could provide a forbidden parameter (e.g. a negative precision) which
would cause a core CRITICAL. We must forbid illegal values from PDB side
(hence outputting a normal plug-in error message, not a core bug).

Also improving a bit the description of this parameter as I was
wondering what precision was needed exactly to get a stroke length. This
is the precision for determining whether a portion of the stroke is
"straight enough" or if we want to break it into smaller pieces until we
get a straight portion.
16 jobs for master in 45 minutes and 52 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Prepare
passed #1098582
build-image

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passed #1098584
build-image-win32

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passed #1098583
build-image-win64

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  Dependencies
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deps-debian

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passed #1098587
deps-win32

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passed #1098586
deps-win64

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  Gimp
passed #1098588
gimp-autotools-debian

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passed #1098591
gimp-clang-debian

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passed #1098589
gimp-distcheck-debian

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passed #1098590
gimp-meson-debian

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passed #1098593
gimp-win32

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passed #1098592
gimp-win64

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  Analysis
passed #1098594
cppcheck

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  Distribution
passed #1098595
sources

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passed #1098597
win32-nightly

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passed #1098596
win64-nightly

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