embed: outdated sentences in documentation

[gazerro]

The documentation of the new embed package says:

The variable must be of type string, []byte, or FS exactly. Named types or type aliases
derived from those types are not allowed.

but the change https://golang.org/cl/282715 removed these limitations.

Similarly, it [the //go:embed directive] can be used with both global and function-local variables,
depending on what is more convenient in context.

but the change https://golang.org/cl/282714 removed support for vars embedded inside functions.

[ianlancetaylor]

Thanks for pointing that out.

[gopherbot]

Change https://golang.org/cl/288072 mentions this issue: embed: update docs for proposal tweaks