[TLS] Added a LangRef entry wrt the module flag MaxTLSAlign.

The module flag was introduced with commit 5d07e04.

llvm/docs/LangRef.rst

@@ -716,7 +716,10 @@ to over-align the global if the global has an assigned section. In this
 case, the extra alignment could be observable: for example, code could
 assume that the globals are densely packed in their section and try to
 iterate over them as an array, alignment padding would break this
-iteration. The maximum alignment is ``1 << 32``.
+iteration. For TLS variables, the module flag ``MaxTLSAlign``, if present,
+limits the alignment to the given value. Optimizers are not allowed to
+impose a stronger alignment on these variables. The maximum alignment
+is ``1 << 32``.
 
 For global variables declarations, as well as definitions that may be
 replaced at link time (``linkonce``, ``weak``, ``extern_weak`` and ``common``