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 <[email protected]>
Reviewed-by: Hyang-Ah Hana Kim <[email protected]>

src/cmd/compile/internal/base/flag.go

@@ -58,7 +58,7 @@ type CmdFlags struct {
  E CountFlag "help:\"debug symbol export\""
  I func(string) "help:\"add `directory` to import search path\""
  K CountFlag "help:\"debug missing line numbers\""
- L CountFlag "help:\"show full file names in error messages\""
+ L CountFlag "help:\"also show actual source file names in error messages for positions affected by //line directives\""
  N CountFlag "help:\"disable optimizations\""
  S CountFlag "help:\"print assembly listing\""
  // V is added by objabi.AddVersionFlag