[LangRef][IR] Fix default AS documentation for allocas without explicit AS (#135942)

So far, the Language Reference said that the alloca address space from
the datalayout is used if no explicit address space is provided, which
is not what the LLParser and the AsmWriter implement. This patch adjusts
the documentation to match the implementation: The default AS 0 is used
if none is explicitly specified.

This is an alternative to PR #135786, which would change the parser's
behavior to match the Language Reference instead.

Author: ritter-x2a

llvm/docs/LangRef.rst

@@ -11104,9 +11104,8 @@ Overview:
 
 The '``alloca``' instruction allocates memory on the stack frame of the
 currently executing function, to be automatically released when this
-function returns to its caller.  If the address space is not explicitly
-specified, the object is allocated in the alloca address space from the
-:ref:`datalayout string<langref_datalayout>`.
+function returns to its caller. If the address space is not explicitly
+specified, the default address space 0 is used.
 
 Arguments:
 """"""""""
@@ -11148,7 +11147,10 @@ which way the stack grows) is not specified.
 
 Note that '``alloca``' outside of the alloca address space from the
 :ref:`datalayout string<langref_datalayout>` is meaningful only if the
-target has assigned it a semantics.
+target has assigned it a semantics. For targets that specify a non-zero alloca
+address space in the :ref:`datalayout string<langref_datalayout>`, the alloca
+address space needs to be explicitly specified in the instruction if it is to be
+used.
 
 If the returned pointer is used by :ref:`llvm.lifetime.start <int_lifestart>`,
 the returned object is initially dead.