Adjust signal on bridge/tunnel map bits, rename accessors, add docs.

Use same bit to store red/green state for entrances and exits.
No longer re-use exit bit to store red/green state of entrance.
Avoid modifying the M2 of tunnel entrances/exits.
Rename and rationalise map accessor functions.
Document bits used in landscape HTML docs.

src/saveload/extended_ver_sl.cpp

@@ -45,7 +45,7 @@ std::vector<uint32> _sl_xv_discardable_chunk_ids;           ///< list of chunks
 static const uint32 _sl_xv_slxi_chunk_version = 0;          ///< current version os SLXI chunk
 
 const SlxiSubChunkInfo _sl_xv_sub_chunk_infos[] = {
-	{ XSLFI_SIG_TUNNEL_BRIDGE,      XSCF_NULL,                2,   2, "signal_tunnel_bridge",      NULL, NULL, NULL        },
+	{ XSLFI_SIG_TUNNEL_BRIDGE,      XSCF_NULL,                3,   3, "signal_tunnel_bridge",      NULL, NULL, NULL        },
 	{ XSLFI_NULL, XSCF_NULL, 0, 0, NULL, NULL, NULL, NULL },// This is the end marker
 };