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.