Improve "self" errors a bit

[GuillaumeGomez]

Currently it looks like this:

let self = process_queue.self();
    ^^^^ `self` value is only available in methods with `self` parameter

Rewording a bit like "self value is a keyword only available in methods with self parameter" would read a bit better and would be more explicit.

[estebank]

Label emitted in the following two places

rust/src/librustc_resolve/lib.rs

Lines 3024 to 3025 in 3bc2ca7

	err.span_label(span, format!("`self` value is only available in \
	methods with `self` parameter"));

The easiest change is rewording as proposed. A harder change would be to make it context dependent and figure out wether it is in happening in a let binding to provide a more tailored message.

[raventid]

Hi, I took an easy path and implemented #54495. If you dislike this approach I can redo this, but It feels to me that taking context in account complicates things, plus I'm not sure about wording.