Docs claim libcore is dependency-free

[hanna-kruppe]

As noted in #29390 among other places, core is not really completely dependency-free. While this claim is defensible in some sense, the crate documentation could be more clear about what it means by this, for example by describing the criterion outlined by @alexcrichton in the aforementioned issue:

[...] having a few small "very well known" functions being unimplemented I think is fine (although they should obviously be minimized where possible).

The docs already talk about mem{cpy,cmp,set} and rust_begin_unwind, but in the meantime libcore gained a few other (intentionally) undefined symbols such as fmod and fmodf --- possibly others as well. The docs should explain those symbols too, and also explain how get around the need for them (don't use % on floats, link with --gc-sections).