Update ST_IsValid docs to state that the function only works with 2D coordinates

[johnpowell]

As can be seen from,

SELECT ST_IsValid('LINESTRING Z (0 0 0,0 0 10)');

NOTICE: Too few points in geometry component at or near point 0 0 0

ST_IsValid ignores the z dimension.

[pramsey]

In 17435:

Add note to valid regarding 2d-only nature
closes #4364