Clarify the doc about GetFeatures.
Clarify the doc about GetFeatures.

Taking the comments from the internal ParseHeadersInternal.
(which is called by GetFeatures).

BUG=webp:411

Change-Id: I9999b4a183805e2db1456610a30024a0d8be4d00

PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp