[NFC] [docs] BitCodeFormat: fill in FUNCTION and USELIST blocks

[woodruffw]

Summary

This fills in a small amount of missing detail in the current bitcode format docs:

• The FUNCTION_BLOCK section is updated to add references to other sub-blocks that can be present
• The USELIST_BLOCK section was added, with stub sub-sections for each record code

[woodruffw]

llvm/docs/BitCodeFormat.rst

1414

N.B.: I don't have enough background on this block to meaningfully fill this section in, so I've uses ellipses like other blocks and codes above.

llvm/docs/BitCodeFormat.rst

 The ``FUNCTION_BLOCK`` block (id 12) ...
 
 In addition to the record types described below, a ``FUNCTION_BLOCK`` block may
 contain the following sub-blocks:
 
 * `CONSTANTS_BLOCK`_
 * `VALUE_SYMTAB_BLOCK`_
 * `METADATA_ATTACHMENT`_
+* `METADATA_BLOCK`_
+* `USELIST_BLOCK`_
 
 .. _VALUE_SYMTAB_BLOCK:
 
 VALUE_SYMTAB_BLOCK Contents
 ---------------------------
 
 The ``VALUE_SYMTAB_BLOCK`` block (id 14) ...
 

 Strings in the string table are not null terminated. A record's *strtab
 offset* and *strtab size* operands specify the byte offset and size of a
 string within the string table.
 
 The string table is used by all preceding blocks in the bitcode file that are
 not succeeded by another intervening ``STRTAB`` block. Normally a bitcode
 file will have a single string table, but it may have more than one if it
 was created by binary concatenation of multiple bitcode files.
+
+.. _USELIST_BLOCK:
+
+USELIST_BLOCK Contents
+----------------------
+
+The ``USELIST_BLOCK`` block (id 18) contains records mapping LLVM values to
+their users.
+
+.. _USELIST_CODE_DEFAULT:
+
+USELIST_CODE_DEFAULT
+^^^^^^^^^^^^^^^^^^^^
+
+``[DEFAULT, index..., value-id]``
+
+The ``DEFAULT`` record (code 1) ...
+
+.. _USELIST_CODE_BB:
+
+USELIST_CODE_BB
+^^^^^^^^^^^^^^^
+
+``[BB, index..., bb-id]``
+
+The ``BB`` record (code 2) ...