Clarify symmetricity of ST_ShiftLongitude in documentation

[lkajan]

The documentation of ST_ShiftLongitude states that the function converts only [-180,180] data to [0,360] but in reality it also converts in reverse.

SELECT ST_AsText(ST_ShiftLongitude('SRID=4326;LINESTRING(179.2 12.2, 180.3 12.8)'::geometry))
-- LINESTRING(179.2 12.2,-179.7 12.8)

SELECT ST_AsText(ST_ShiftLongitude('SRID=4326;LINESTRING(179.2 12.2,-179.7 12.8)'::geometry))
-- LINESTRING(179.2 12.2,180.3 12.8)

[lkajan]

Pull request: ​https://github.com/postgis/postgis/pull/642

[leoregina <lr@…>]

In 85553d7e/git:

Merge remote-tracking branch 'Kajan/shiftlongitude-doc'
References #5010

[Regina Obe <lr@…>]

In f42489b/git:

Doc clarifications for ST_ShiftLongitude
from Lauri Kajan
Closes #5010