Add attributes.codegen.inline.externally-exported

This rule did not seem to be covered elsewhere. I'm not 100% positive
this is the best way to document this, but I think it is accurate.

This is implemented in
https://github.com/rust-lang/rust/blob/5526a2f47cd676ceeedc08cf71ae75ce2e9284ae/compiler/rustc_passes/src/check_attr.rs#L482-L497

Author: ehuss

src/attributes/codegen.md

@@ -65,6 +65,9 @@ There are three ways to use the inline attribute:
 r[attributes.codegen.inline.trait]
 When `inline` is applied to a function in a [trait definition], it applies only to that function when used as a default function for a trait implementation and not to all trait implementations.
 
+r[attributes.codegen.inline.externally-exported]
+The `inline` attribute is ignored if the function is externally exported. This may happen with the [`no_mangle`] or [`export_name`] attribute.
+
 r[attributes.codegen.cold]
 ### The `cold` attribute