In the process also document that new `--remap-path-scope` scopes may be
added in the future, and that the `all` scope always represent all the
scopes.
Co-authored-by: David Wood <agile.lion3441@fuligin.ink>
Previously in the `//`-compiletest-directive times, this was implemented
as a special `no-*` directive parsing. In the migration from `//` ->
`//@`, the `// no-remap-src-base` directive was lost, most likely
because it had no effect -- the default is not remapping `src-base`.
So remove occurrences of `no-remap-src-base`, as these are not valid
directives.
