Document that XS calling to other XS functions that modify the stack should PUTBACK/SPAGAIN

[wolfsage]

It would seem that if you have:
void x():
...
call_sv(...)
...

void y():
    ....
    x();

That y() must wrap x() with a PUTBACK/SPAGAIN, otherwise the stack might be corrupted.

This does not seem to be documented anywhere, perhaps add a note within perlcall()...

[bulk88]

This is referring to stack reallocation. If a Xsub/func C has an auto SP/dSP; and calls another directly or indirectly (unlimited stack frame depth) a C func that does a call_*, the "Xsub/func C has an auto SP/dSP;" must SPAGAIN;