[LangRef] Clarify support for multiple metadata attachments with same id

As discussed on D105251, currently the compiler does not support
multiple metadata attachments on instructions having the same
identifier, whereas it does for global objects. Note this in the
Language Reference manual for clarity.

See D105251 for discussions of history behind this divergence, and the
complexities and possible approaches of adding this support to
instructions in the future.

Differential Revision: https://reviews.llvm.org/D106304

Author: teresajohnson

llvm/docs/LangRef.rst

@@ -5035,8 +5035,8 @@ occurs on.
 Metadata
 ========
 
-LLVM IR allows metadata to be attached to instructions in the program
-that can convey extra information about the code to the optimizers and
+LLVM IR allows metadata to be attached to instructions and global objects in the
+program that can convey extra information about the code to the optimizers and
 code generator. One example application of metadata is source-level
 debug information. There are two metadata primitives: strings and nodes.
 
@@ -5096,6 +5096,9 @@ to the ``add`` instruction using the ``!dbg`` identifier:
 
     %indvar.next = add i64 %indvar, 1, !dbg !21
 
+Instructions may not have multiple metadata attachments with the same
+identifier.
+
 Metadata can also be attached to a function or a global variable. Here metadata
 ``!22`` is attached to the ``f1`` and ``f2`` functions, and the globals ``g1``
 and ``g2`` using the ``!dbg`` identifier:
@@ -5110,6 +5113,9 @@ and ``g2`` using the ``!dbg`` identifier:
     @g1 = global i32 0, !dbg !22
     @g2 = external global i32, !dbg !22
 
+Unlike instructions, global objects (functions and global variables) may have
+multiple metadata attachments with the same identifier.
+
 A transformation is required to drop any metadata attachment that it does not
 know or know it can't preserve. Currently there is an exception for metadata
 attachment to globals for ``!type`` and ``!absolute_symbol`` which can't be