apps/app_amd: Document maximum_word_length option; fix AMDCAUSE documentation

apps/app_amd: Document maximum_word_length option; fix AMDCAUSE documentation

This patch corrects the documentation for the AMD application. Specifically:
* It documents the maximum_word_length option, which limits the maximum allowed
  length of a single utterance.
* It clarifies the AMDCAUSE values MAXWORDS and MAXWORDLENGTH. MAXWORDLENGTH
  was documented as MAXWORDS, while MAXWORDS was undocumented.

Thanks to the issue reporter, Frank DiGennaro, for pointing out the issues.

ASTERISK-19470 #close
Reported by: Frank DiGennaro

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@432918 65c4cc65-6c06-0410-ace0-fbb531ad65f3