cmd/compile: clarify help message for -L flag
cmd/compile: clarify help message for -L flag

The file names reported in error messages by the compiler are
printed unchanged from the file names provided to the compiler;
the -L flag has no impact on the file names themselves, contrary
to what the old flag description suggested.

If an error is reported on a line that is affected by a //line
directive, an error message reports the file name and line as
controlled by the directive (i.e., the actual source position
is not known).

If the -L flag is provided, the actual source position is also
reported in square brackets.

This change documents this with an updated help string for the
flag.

For #36988.

Change-Id: I39ee35e6ff6cd5cfa44d87dabb05b8d78575d631
Reviewed-on: https://go-review.googlesource.com/c/go/+/395115
Trust: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>

CAFxX/go,golang/go,golang/go,nawawi/go,CAFxX/go,nawawi/go,CAFxX/go,CAFxX/go,CAFxX/go,CAFxX/go,golang/go,CAFxX/go,nawawi/go,nawawi/go,nawawi/go,golang/go,CAFxX/go,golang/go,nawawi/go,golang/go,nawawi/go,golang/go,nawawi/go,golang/go