Add export_name to the attribute whitelist and a description in the rust manual

Author: jackheizer

src/doc/rust.md

@@ -1928,6 +1928,8 @@ interpreted:
 
 ### Miscellaneous attributes
 
+- `export_name` - on statics and functions, this determines the name of the
+  exported symbol.
 - `link_section` - on statics and functions, this specifies the section of the
   object file that this item's contents will be placed into.
 - `macro_export` - export a macro for cross-crate usage.

src/librustc/lint/builtin.rs

@@ -568,6 +568,7 @@ impl LintPass for UnusedAttribute {
             // FIXME: #14406 these are processed in trans, which happens after the
             // lint pass
             "cold",
+            "export_name",
             "inline",
             "link",
             "link_name",