[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