doc: Document -O1 as the preferred level for large machine-generated code

doc: Document -O1 as the preferred level for large machine-generated code

At -O1, the intention is that we compile things in a "reasonable" amount
of time (ditto memory use). In particular, we try to especially avoid
optimizations which scale poorly on pathological cases, as is the case
for large machine-generated code.

Recommend -O1 for large machine-generated code, as has been informally
done on bugs for a while now.

This applies (broadly speaking) for both large machine-generated functions
but also to a lesser extent repetitive small-but-still-not-tiny functions
from a generator program.

gcc/ChangeLog:
	PR middle-end/114855
	* doc/invoke.texi (Optimize options): Mention machine-generated
	code for -O1.