Fix regression since commit:
915807f8db "Rename doc/mapformat.txt and update doc to match SRP changes"
And add an underscore to doc file name to make it more readable.
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.
The documentation file contains not just information about the
map itself, but also about further files inside the world's
directory.
Documentation didn't reflect recent SRP addition, now it does.