Add help info for --enable-vp9-highbitdepth

This fixes issue# 903.

Change-Id: I649d8584898a93681073e1ac1b444ff66e240050

configure

@@ -36,6 +36,7 @@ Advanced options:
   ${toggle_codec_srcs}            in/exclude codec library source code
   ${toggle_debug_libs}            in/exclude debug version of libraries
   ${toggle_static_msvcrt}         use static MSVCRT (VS builds only)
+  ${toggle_vp9_highbitdepth}      use VP9 high bit depth (10/12) profiles
   ${toggle_vp8}                   VP8 codec support
   ${toggle_vp9}                   VP9 codec support
   ${toggle_internal_stats}        output of encoder internal stats for debug, if supported (encoders)