[llvm] [docs][PDB] Document niMac after Named Stream Map (PR #214689)

[Alexandre Ganea via llvm-commits]

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@@ -93,6 +93,12 @@ The on-disk layout of the serialized hash table is described at :doc:`HashTable`
 Note that the entire Named Stream Map is not length-prefixed, so the only way to
 get to the data following it is to de-serialize it in its entirety.
 
+After the serialized hash table, producers append a 32-bit ``niMac`` field used
+by Microsoft's named-stream table implementation when assigning stream name
+indices. Consumers that only need the name-to-index map may ignore this value,
+but parsers that read past the map must account for these 4 bytes before the
+feature codes that follow.
+
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aganea wrote:

There's a possibly an unwanted `\n` here.

https://github.com/llvm/llvm-project/pull/214689