[perl #80426] Document usage of strlen() by is_utf8_string() and others

# New Ticket Created by  Marvin Humphrey 
# Please include the string:  [perl #80426]
# in the subject line of all future correspondence about this issue. 
# <URL: http://rt.perl.org/rt3/Ticket/Display.html?id=80426 >


If the 'len' parameter to is_utf8_string() is 0, strlen() will be invoked on
the supplied 's' parameter to calculate it.  The same is true for
is_ascii_string(), is_utf8_string_loc(), and is_utf8_string_loclen().  This
behavior has existed for years, but is not documented in perlapi.  

The supplied patch amends the documentation for is_utf8_string() and
is_ascii_string(); the other functions inherit their documentation from
is_utf8_string() and thus do not require changes.

• [perl #80426] Document usage of strlen() by is_utf8_string() and others by Father Chrysostomos via RT
• [perl #80426] Document usage of strlen() by is_utf8_string() and others by Marvin Humphrey