doc/rust.md: Demonstrate the `f::<T>()` syntax more often

doc/rust.md: Demonstrate the `f::<T>()` syntax more often

The "Generic functions" subsection mentions that generic functions are
instantiated based on context, so let's also mention right away (with a
link to the #paths section) that an explicit form is available.

This also adds an example to the function call expression section that
explicitly instantiates a generic function.