Add libcore documentation for macros::builtin (include_bytes!, include_str!, etc.)

[newpavlov]

While include_bytes!, include_str! and other macros from std::macros::builtin are available with no_std there is no documentation for them in libcore docs.

[TimNN]

Are you sure they are not available with no_std? As far as I know these are built into the compiler and available in all crates. I think it is only their documentation which is only available in std and not in core.

This should be true for all macros in the std::macros::builtin module.

[newpavlov]

Hm, they are indeed available. I will edit this issue.