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

[tejohnson]

Summary

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.

[dexonsmith]

LGTM; thanks!

llvm/docs/LangRef.rst

 will use the one that corresponds to the line of the asm that the error
 occurs on.
 
 .. _metadata:
 
 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.
 
 Metadata does not have a type, and is not a value. If referenced from a
 ``call`` instruction, it uses the ``metadata`` type.
 
 All metadata are identified in syntax by an exclamation point ('``!``').
 

 
 Metadata can be attached to an instruction. Here metadata ``!21`` is attached
 to the ``add`` instruction using the ``!dbg`` identifier:
 
 .. code-block:: llvm
 
     %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:
 
 .. code-block:: llvm
 
     declare !dbg !22 void @f1()
     define void @f2() !dbg !22 {
       ret void
     }
 
     @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
 unconditionally dropped unless the global is itself deleted.
 
 Metadata attached to a module using named metadata may not be dropped, with
 the exception of debug metadata (named metadata with the name ``!llvm.dbg.*``).