gas: document that get_symbol_name() can clobber the input buffer

gas: document that get_symbol_name() can clobber the input buffer

Callers which want to make further parsing attempts at the buffer passed
to the function need to be aware that due to the potential of string
concatenation the input buffer may be altered in ways beyond what can be
undone by putting back at *input_line_pointer the character that the
function returns.