Add options in create_captcha() to specify the randomly generated captcha word length and character pool

Uses the same defaults as were hard coded in (8 chars in length,
0-9a-aA-Z). Small change in this file means less code elsewhere when
generating random character strings for the captcha word.
typo change

Uses the same defaults as were hard coded in (8 chars in length,
0-9a-aA-Z). Small change in this file means less code elsewhere when
generating random character strings for the captcha word. all existing
code will work the same.
Updated documenation
documenation edit
Documentation change - explanation of captcha_word_length and character_pool_for_generated_word
removed sanity checks (developer-supplied value, not user input), added changelog entry, changed variable names
changed changelog format.
changelog change
eol test.
eol 2
eol
newline at end
user guide newline.
final change (extra space)
Merge pull request #2396 from abkr/develop

Add options in create_captcha() to specify the randomly generated captcha word length and character pool