[LangRef] document override-stack-alignment module flag

[nickdesaulniers]

Summary

This module flag was added in D103048.

[tejohnson]

llvm/docs/LangRef.rst

7836

Nit: s/values/value/ to go with "an i32".

Also, IIRC it is an error to have this module flag specified with a different value by different modules - should that be noted here?

[nickdesaulniers]

• singular, add note about error

[tejohnson]

lgtm

llvm/docs/LangRef.rst

 
 LTO Post-Link Module Flags Metadata
 -----------------------------------
 
 Some optimisations are only when the entire LTO unit is present in the current
 module. This is represented by the ``LTOPostLink`` module flags metadata, which
 will be created with a value of ``1`` when LTO linking occurs.
 
+Stack Alignment Metadata
+------------------------
+
+Changes the default stack alignment from the target ABI's implicit default
+stack alignment. Takes an i32 value in bytes. It is considered an error to link
+two modules together with different values for this metadata.
+
+For example:
+
+    !llvm.module.flags = !{!0}
+    !0 = !{i32 1, !"override-stack-alignment", i32 8}
+
+This will change the stack alignment to 8B.
+
 Embedded Objects Names Metadata
 ===============================
 
 Offloading compilations need to embed device code into the host section table to
 create a fat binary. This metadata node references each global that will be
 embedded in the module. The primary use for this is to make referencing these
 globals more efficient in the IR. The metadata references nodes containing
 pointers to the global to be embedded followed by the section name it will be