add descriptive docs for mailmap check

Co-authored-by: Emily <github@emily.moe>

Author: Byron

src/plumbing/options/mod.rs

@@ -518,7 +518,7 @@ pub mod mailmap {
     pub enum Subcommands {
         /// Print all entries in configured mailmaps, inform about errors as well.
         Entries,
-        /// Print all entries in configured mailmaps, inform about errors as well.
+        /// Print the canonical form of contacts according to the configured mailmaps.
         Check {
             /// One or more `Name <email>` or `<email>` to pass through the mailmap.
             contacts: Vec<BString>,