Serialisation: documentation fixes, clarifying renames and whitespace fixes

1. Do two renames:
	* SER_FMT_CLIENT_VER_LOWEST -> SER_FMT_VER_LOWEST_WRITE
	* SER_FMT_VER_LOWEST -> SER_FMT_VER_LOWEST_READ
Now the two define values are consistently named with the _WRITE defines
SER_FMT_VER_{HIGHEST,LOWEST}_WRITE, and to better point out what the two
serialisation versions actually are for.

2. wrap some lines in doc/worldformat.txt, and point out that the node
timers are serialized at a later point, as this can cause confusion about
what now happens (if one doesn't strictly read the if block's conditions).

3. some whitespace fixes in NodeTimerList::serialize, and one new comment.

src/mapblock.cpp

@@ -564,7 +564,7 @@ void MapBlock::serialize(std::ostream &os, u8 version, bool disk)
 		throw SerializationError("ERROR: Not writing dummy block.");
 	}
 
-	FATAL_ERROR_IF(version < SER_FMT_CLIENT_VER_LOWEST, "Serialize version error");
+	FATAL_ERROR_IF(version < SER_FMT_VER_LOWEST_WRITE, "Serialisation version error");
 
 	// First byte
 	u8 flags = 0;