pbx_variables: Increase parsing capabilities of MSet

Currently MSet can only parse a maximum of 24 variables.
If more variables are provided to MSet, the 24th variable
will simply contain the remainder of the string and the
remaining variables thereafter will never get set.

This increases the number of variables that can be parsed
in one go from 24 to 99. Additionally, documentation is added
since this limitation is currently undocumented and is
confusing to users who encounter this limitation.

ASTERISK-29766 #close

Change-Id: I3fe35b462dedec0a452fd9ea7f92c920a3939f16