Rename 'matlab-incompatible' warning to 'language-extension'. * libinterp/corefcn/error.cc: replace the 'matlab-incompatible' warning with 'language-extension'. This wording will hopefully make it clear that it does not stop or prevent Matlab code from running correctly in Octave, it simply warns about usage of Octave only features. * libinterp/parse-tree/lex.h, libinterp/parse-tree/lex.ll: throw the new 'language-extension' warning ID. Also, adjust function names to better reflect the warning name. * libinterp/corefcn/gripes.cc, libinterp/octave-value/ov-struct.cc, liboctave/numeric/bsxfun.h: use the new 'language-extension' warning ID. * scripts/miscellaneous/warning_ids.m: fix warning ID and reword text. * doc/interpreter/basics.txi, doc/interpreter/container.txi, scripts/miscellaneous/setfield.m: replace warning ID on documentation. * NEWS: make note of the change.