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
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Merged revisions 432918 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 432920 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@432921 65c4cc65-6c06-0410-ace0-fbb531ad65f3

apps/app_amd.c

@@ -126,6 +126,9 @@ ASTERISK_FILE_VERSION(__FILE__, "$Revision$")
 						Voice Duration - Greeting.
 					</value>
 					<value name="MAXWORDLENGTH">
+						Word Length - max length of a single word.
+					</value>
+					<value name="MAXWORDS">
 						Word Count - maximum number of words.
 					</value>	
 				</variable>

configs/samples/amd.conf.sample

@@ -15,4 +15,5 @@ between_words_silence = 50	; Minimum duration of silence after a word to conside
 				; the audio what follows as a new word
 maximum_number_of_words = 3	; Maximum number of words in the greeting.
 				; If exceeded then MACHINE
+maximum_word_length = 5000      ; Maximum duration of a single Voice utterance allowed.
 silence_threshold = 256