[LangRef] Update new ssp/sspstrong/sspreq semantics after D91816

[MaskRay]

Fix a typo

[nickdesaulniers]

Thanks for the patch!

[MaskRay]

Mention alwaysinline

[MaskRay]

Clarify

llvm/docs/LangRef.rst

``ssp``
     - Character arrays larger than ``ssp-buffer-size`` (default 8).
     - Aggregates containing character arrays larger than ``ssp-buffer-size``.
     - Calls to alloca() with variable sizes or constant sizes greater than
       ``ssp-buffer-size``.
 
     Variables that are identified as requiring a protector will be arranged
     on the stack such that they are adjacent to the stack protector guard.
 
-    If a function that has an ``ssp`` attribute is inlined into a
-    function that doesn't have an ``ssp`` attribute, then the resulting
-    function will have an ``ssp`` attribute.
+    A function with the ``ssp`` attribute but without the ``alwaysinline``
+    attribute cannot be inlined into a function without a
+    ``ssp/sspreq/sspstrong`` attribute.
 ``sspreq``
-    This attribute indicates that the function should *always* emit a
-    stack smashing protector. This overrides the ``ssp`` function
-    attribute.
+    This attribute indicates that the function should *always* emit a stack
+    smashing protector. This overrides the ``ssp`` and ``sspstrong`` function
+    attributes.
 
     Variables that are identified as requiring a protector will be arranged
     on the stack such that they are adjacent to the stack protector guard.
     The specific layout rules are:
 
     #. Large arrays and structures containing large arrays
        (``>= ssp-buffer-size``) are closest to the stack protector.
     #. Small arrays and structures containing small arrays
        (``< ssp-buffer-size``) are 2nd closest to the protector.
     #. Variables that have had their address taken are 3rd closest to the
        protector.
 
-    If a function that has an ``sspreq`` attribute is inlined into a
-    function that doesn't have an ``sspreq`` attribute or which has an
-    ``ssp`` or ``sspstrong`` attribute, then the resulting function will have
-    an ``sspreq`` attribute.
+    A function with the ``sspreq`` attribute but without the ``alwaysinline``
+    attribute cannot be inlined into a function without a
+    ``ssp/sspstrong/sspreq`` attribute. If a function with the ``sspreq``
+    attribute is inlined into a function with a ``ssp/sspstrong`` attribute,
+    the attribute in the caller will upgrade to ``sspreq``.
 ``sspstrong``
     This attribute indicates that the function should emit a stack smashing
     protector. This attribute causes a strong heuristic to be used when
     determining if a function needs stack protectors. The strong heuristic
     will enable protectors for functions with:
 
     - Arrays of any size and type
     - Aggregates containing an array of any size and type.
     - Calls to alloca().
     - Local variables that have had their address taken.
 
     Variables that are identified as requiring a protector will be arranged
     on the stack such that they are adjacent to the stack protector guard.
     The specific layout rules are:
 
     #. Large arrays and structures containing large arrays
        (``>= ssp-buffer-size``) are closest to the stack protector.
     #. Small arrays and structures containing small arrays
        (``< ssp-buffer-size``) are 2nd closest to the protector.
     #. Variables that have had their address taken are 3rd closest to the
        protector.
 
     This overrides the ``ssp`` function attribute.
 
-    If a function that has an ``sspstrong`` attribute is inlined into a
-    function that doesn't have an ``sspstrong`` attribute, then the
-    resulting function will have an ``sspstrong`` attribute.
+    A function with the ``sspstrong`` attribute but without the
+    ``alwaysinline`` attribute cannot be inlined into a function without a
+    ``ssp/sspstrong/sspreq`` attribute. If a function with the ``sspstrong``
+    attribute is inlined into a function with the ``ssp`` attribute, the
+    attribute in the caller will upgrade to ``sspstrong``.
 ``strictfp``
     This attribute indicates that the function was called from a scope that
     requires strict floating-point semantics.  LLVM will not attempt any
     optimizations that require assumptions about the floating-point rounding
     mode or that might alter the state of floating-point status flags that
     might otherwise be set or cleared by calling this function. LLVM will
     not introduce any new floating-point instructions that may trap.