Document RPMDBI_SIGMD5 and RPMDBI_SHA1HEADER as obsolete

Nothing and nobody should be using these for anything at all.
We'd rather remove these indexes entirely but doing so while keeping
compatibility with existing databases and so on is far more trouble
than letting these die of old age is. V6 packages will not have such
tags so in time these indexes will be all empty and then we can just
axe them without having to work out transitional details.

Related: #2633

include/rpm/rpmtag.h

@@ -419,8 +419,8 @@ typedef enum rpmDbiTag_e {
     RPMDBI_TRIGGERNAME		= RPMTAG_TRIGGERNAME,
     RPMDBI_DIRNAMES		= RPMTAG_DIRNAMES,
     RPMDBI_INSTALLTID		= RPMTAG_INSTALLTID,
-    RPMDBI_SIGMD5		= RPMTAG_SIGMD5,
-    RPMDBI_SHA1HEADER		= RPMTAG_SHA1HEADER,
+    RPMDBI_SIGMD5		= RPMTAG_SIGMD5,	/* OBSOLETE */
+    RPMDBI_SHA1HEADER		= RPMTAG_SHA1HEADER,	/* OBSOLETE */
     RPMDBI_INSTFILENAMES	= RPMTAG_INSTFILENAMES,
     RPMDBI_FILETRIGGERNAME	= RPMTAG_FILETRIGGERNAME,
     RPMDBI_TRANSFILETRIGGERNAME = RPMTAG_TRANSFILETRIGGERNAME,