[clang] Note that optnone and target attributes do not apply to nested functions (#82815)

This behavior is true for all attributes, but this behavior can be
surprising for attributes which have function-wide effects, such as
`optnone` and `target`. Most other function attributes affect the
prototype or semantics, but do not affect code generation in the
function body. I believe it is worth calling this out in the
documentation of these function-wide attributes. There may be more,
these were the two that came to mind.

Author: rnk

clang/include/clang/Basic/AttrDocs.td

@@ -2527,6 +2527,9 @@ Example "subtarget features" from the x86 backend include: "mmx", "sse", "sse4.2
 "avx", "xop" and largely correspond to the machine specific options handled by
 the front end.
 
+Note that this attribute does not apply transitively to nested functions such
+as blocks or C++ lambdas.
+
 Additionally, this attribute supports function multiversioning for ELF based
 x86/x86-64 targets, which can be used to create multiple implementations of the
 same function that will be resolved at runtime based on the priority of their
@@ -3782,6 +3785,12 @@ for that function.
 
 This attribute is incompatible with the ``always_inline`` and ``minsize``
 attributes.
+
+Note that this attribute does not apply recursively to nested functions such as
+lambdas or blocks when using declaration-specific attribute syntaxes such as double
+square brackets (``[[]]``) or ``__attribute__``. The ``#pragma`` syntax can be
+used to apply the attribute to all functions, including nested functions, in a
+range of source code.
   }];
 }