Add document for member_name

Author: Kai Luo

src/symbolize/gimli.rs

@@ -278,6 +278,11 @@ struct Cache {
 struct Library {
     name: OsString,
     #[cfg(target_os = "aix")]
+    /// On AIX, the library mmapped can be a member of an big-archive file.
+    /// For example, an big-archive named libfoo.a contains libbar.so,
+    /// one can use `dlopen("libfoo.a(libbar.so)", RTLD_MEMBER | RTLD_LAZY)`
+    /// to use the `libbar.so` library. In this case, only `libbar.so` is
+    /// mmapped, not the whole `libfoo.a`.
     member_name: OsString,
     /// Segments of this library loaded into memory, and where they're loaded.
     segments: Vec<LibrarySegment>,