cmd/go: clarify -dropreplace help text in 'go help mod edit'

Fixes #34930

Change-Id: Ie0ddd8de6e40f5a192c4d2267399126e449fe5f5
Reviewed-on: https://go-review.googlesource.com/c/go/+/210340
Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

Author: Jay Conrod

src/cmd/go/alldocs.go

@@ -55,12 +55,17 @@ The -exclude=path@version and -dropexclude=path@version flags
 add and drop an exclusion for the given module path and version.
 Note that -exclude=path@version is a no-op if that exclusion already exists.
 
-The -replace=old[@v]=new[@v] and -dropreplace=old[@v] flags
-add and drop a replacement of the given module path and version pair.
-If the @v in old@v is omitted, the replacement applies to all versions
-with the old module path. If the @v in new@v is omitted, the new path
-should be a local module root directory, not a module path.
-Note that -replace overrides any existing replacements for old[@v].
+The -replace=old[@v]=new[@v] flag adds a replacement of the given
+module path and version pair. If the @v in old@v is omitted, a
+replacement without a version on the left side is added, which applies
+to all versions of the old module path. If the @v in new@v is omitted,
+the new path should be a local module root directory, not a module
+path. Note that -replace overrides any redundant replacements for old[@v],
+so omitting @v will drop existing replacements for specific versions.
+
+The -dropreplace=old[@v] flag drops a replacement of the given
+module path and version pair. If the @v is omitted, a replacement without
+a version on the left side is dropped.
 
 The -require, -droprequire, -exclude, -dropexclude, -replace,
 and -dropreplace editing flags may be repeated, and the changes