ST_OffsetCurve Documentation should explicitly mention variable return type

[gturner]

In 99% of cases, ST_OffsetCurve returns a LineString, and this is the intuitive return type. However, in the remaining 1% of cases, it in fact returns a MultiLineString. (The fact that some of those appear to be unjustifiably so is probably another ticket!) In many cases, programmers would simply assign the output to a linestring. This will cause intermittent bugs when a Multilinestring is returned. The documentation should explicitly say that this may be the case, allowing a programmer to allow for that possibility. It would be good to illustrate this possibility with an example, such as a jigsaw-tongue type structure where the width of the narrow part of the tongue is less than twice the offset distance.

[robe]

I may push this back if I have time or to 2.1.3 -- just getting it out of the 2.1.2 queue so we can release soon.

[komzpa]

In 17675:

[doc] Note about mulitlinestring in ST_OffsetCurve
Closes #2553