Changes to [0] which worked as intended but used confusing naming.
- The note on using -1 for the active layer causes an assertion.
- The doc-string was above the wrong function.
- The meaning of the `_n()` suffix was flipped,
where the `layer_index_n` refers to an absolute index across all
layer types which is done internally for an index calculated from the
`layer` argument, not the argument it's self which is a UV index.
Rename BM_uv_map_get_offsets_n to BM_uv_map_get_offsets_from_layer.
[0]: 412b6a8f65
26aa1b1367