Documentation/gitformat-pack.txt: fix incorrect MIDX documentation

Back in 32f3c54 (multi-pack-index: write pack names in chunk,
2018-07-12) the MIDX's "Packfile Names" (or "PNAM", for short) chunk was
described as containing an array of string entries. e0d1bcf notes
that this is the only chunk in the MIDX format's specification that is
not guaranteed to be 4-byte aligned, and so should be placed last.

This isn't quite accurate: the entries within the PNAM chunk are not
guaranteed to be 4-byte aligned since they are arbitrary strings, but
the chunk itself is 4-byte aligned since the ending is padded with NUL
bytes.

That padding has always been there since 32f3c54 via
midx.c::write_midx_pack_names(), which ended with:

    i = MIDX_CHUNK_ALIGNMENT - (written % MIDX_CHUNK_ALIGNMENT)
    if (i < MIDX_CHUNK_ALIGNMENT) {
      unsigned char padding[MIDX_CHUNK_ALIGNMENT];
      memset(padding, 0, sizeof(padding))
      hashwrite(f, padding, i);
      written += i;
    }

In fact, 32f3c54's log message itself describes the chunk in its
first paragraph with:

    Since filenames are not well structured, add padding to keep good
    alignment in later chunks.

So these have always been externally aligned. Correct the corresponding
part of our documentation to reflect that.

Helped-by: Junio C Hamano <[email protected]>
Signed-off-by: Taylor Blau <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: ttaylorr

Documentation/gitformat-pack.txt

@@ -390,10 +390,11 @@ CHUNK LOOKUP:
 CHUNK DATA:
 
 	Packfile Names (ID: {'P', 'N', 'A', 'M'})
-	    Stores the packfile names as concatenated, NUL-terminated strings.
-	    Packfiles must be listed in lexicographic order for fast lookups by
-	    name. This is the only chunk not guaranteed to be a multiple of four
-	    bytes in length, so should be the last chunk for alignment reasons.
+	    Store the names of packfiles as a sequence of NUL-terminated
+	    strings. There is no extra padding between the filenames,
+	    and they are listed in lexicographic order. The chunk itself
+	    is padded at the end with between 0 and 3 NUL bytes to make the
+	    chunk size a multiple of 4 bytes.
 
 	OID Fanout (ID: {'O', 'I', 'D', 'F'})
 	    The ith entry, F[i], stores the number of OIDs with first