Documentation: deltaBaseCacheLimit is per-thread

Clarify that core.deltaBaseCacheLimit is per-thread, as can be seen from
the fact that cache usage (base_cache_used in struct thread_local in
builtin/index-pack.c) is tracked individually for each thread and
compared against delta_base_cache_limit.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/core.txt

@@ -399,7 +399,7 @@ the largest projects.  You probably do not need to adjust this value.
 Common unit suffixes of 'k', 'm', or 'g' are supported.
 
 core.deltaBaseCacheLimit::
-	Maximum number of bytes to reserve for caching base objects
+	Maximum number of bytes per thread to reserve for caching base objects
 	that may be referenced by multiple deltified objects.  By storing the
 	entire decompressed base objects in a cache Git is able
 	to avoid unpacking and decompressing frequently used base