Many functions in string.c assume that capa + termlen to be readable
memory. Add comment in header to communicate this to extension authors.
See also: comment in str_fill_term()
https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html says:
> GCC never returns 1 when you call the inline function with a string
> constant or ...
Because rb_str_new and familiy take string constants, it was a bad idea
for them to contain RB_CONSTANT_P. We should instead do so in
corresponding macros. [ruby-core:98212]
