find_library: improve the docs and FeatureNew to fully describe the change

The docs didn't really explain what the issue was with using it. And
it's not actually a "crash" either way.

The FeatureNew mentions that "name" is new, but it is standard for
these warnings to tell you both the type of object you're operating on
and the name of the method that is an issue. This omitted the former,
and was very confusing.

docs/markdown/snippets/find_library_name.md

@@ -1,3 +1,5 @@
 ## dependencies created by compiler.find_library implement the `name()` method
 
-Which would previously result in Meson crashing.
+Previously, for a [[dep]] that might be returned by either [[dependency]] or
+[[compiler.find_library]], the method might or might not exist with no way
+of telling.

mesonbuild/interpreter/interpreterobjects.py

@@ -664,7 +664,7 @@ class ExternalLibraryHolder(ObjectHolder[ExternalLibrary]):
         pdep = self.held_object.get_partial_dependency(**kwargs)
         return pdep
 
-    @FeatureNew('name', '1.5.0')
+    @FeatureNew('dependency.name', '1.5.0')
     @noPosargs
     @noKwargs
     def name_method(self, args: T.List[TYPE_var], kwargs: TYPE_kwargs) -> str: