[LangRef][DebugInfo] Update DIFile checksum description

[jryans]

Summary

DIFile's checksum fields became optional as part of
https://reviews.llvm.org/D43043. This updates the description in the docs to
match.

Fixes https://github.com/llvm/llvm-project/issues/39151

[scott.linder]

LGTM, thank you for the fix!

llvm/docs/LangRef.rst

 .. code-block:: none
 
     !0 = !DIFile(filename: "path/to/file", directory: "/path/to/dir",
                  checksumkind: CSK_MD5,
                  checksum: "000102030405060708090a0b0c0d0e0f")
 
 Files are sometimes used in ``scope:`` fields, and are the only valid target
 for ``file:`` fields.
-Valid values for ``checksumkind:`` field are: {CSK_None, CSK_MD5, CSK_SHA1, CSK_SHA256}
+
+The ``checksum:`` and ``checksumkind:`` fields are optional. If one of these
+fields is present, then the other is required to be present as well. Valid
+values for ``checksumkind:`` field are: {CSK_MD5, CSK_SHA1, CSK_SHA256}
 
 .. _DIBasicType:
 
 DIBasicType
 """""""""""
 
 ``DIBasicType`` nodes represent primitive types, such as ``int``, ``bool`` and
 ``float``. ``tag:`` defaults to ``DW_TAG_base_type``.