doc: technical details about the index file format

doc: technical details about the index file format

 * Clarify "string of unsigned bytes";

 * Blob has two variants (regular file vs symlink), not (blob vs symlink);

 * Clarify permission mode bits;

 * Clarify ce_namelen() "too long to fit in the length field" case;

 * Clarify "." etc are forbidden as path components;

 * Match the description with the internal wording "cache-tree";

 * All types of extension begin with signature and length as explained in
   the first part. Don't repeat the "length" part in the description of
   each extension (can be mistaken as if there is a separate 32-bit size
   field inside the extension), but state what the signature for each
   extension is.

 * Don't say "Extension tag", as we have said "Extension signature" in the
   first part---be consistent;

 * Clarify the invalidation of cache-tree entries;

 * Correct description on subtree_nr field in the cache-tree;

 * Clarify the order of entries in cache-tree;

Signed-off-by: Junio C Hamano <gitster@pobox.com>