[LangRef] Patchable function attributes (#195764)

Langref should document the expected layout for patchable-function-entry
/ patchable-function-prefix attrs (prefix nops go before the function
label, entry nops go after the label, both are before the prologue).

Fixes #195693.

llvm/docs/LangRef.rst

diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index 88f5028..83bd6c7 100644
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -2502,6 +2502,17 @@
     This attribute by itself does not imply restrictions on
     inter-procedural optimizations.  All of the semantic effects the
     patching may have to be separately conveyed via the linkage type.
+``"patchable-function-prefix"``
+    This attribute specifies the number of target-specific NOP instructions
+    emitted before the function entry label.
+``"patchable-function-entry"``
+    This attribute specifies the number of target-specific NOP instructions
+    emitted after the function entry label.  These NOPs are emitted before the
+    function prologue.
+``"patchable-function-entry-section"``
+    This attribute specifies the section used to record the start of the
+    patchable function entry area when such a section is emitted.  If omitted,
+    the default section name is ``__patchable_function_entries``.
 ``"probe-stack"``
     This attribute indicates that the function will trigger a guard region
     in the end of the stack. It ensures that accesses to the stack must be