DOC: correct outdated comment in matplotlibrc

Technically ffmpeg will stitch a series of different size static images into a
movie, however it does so my scaling each subsequent frame to be the dimensions
of the first which results in sub-optimal output.

Author: tacaswell

lib/matplotlib/mpl-data/matplotlibrc

@@ -690,9 +690,8 @@
 #savefig.edgecolor: auto        # figure edge color when saving
 #savefig.format:    png         # {png, ps, pdf, svg}
 #savefig.bbox:      standard    # {tight, standard}
-                                # 'tight' is incompatible with pipe-based animation
-                                # backends (e.g. 'ffmpeg') but will work with those
-                                # based on temporary files (e.g. 'ffmpeg_file')
+                                # 'tight' is incompatible with generating frames
+                                # for animation
 #savefig.pad_inches:  0.1       # padding to be used, when bbox is set to 'tight'
 #savefig.directory:   ~         # default directory in savefig dialog, gets updated after
                                 # interactive saves, unless set to the empty string (i.e.