Fix markdown, use multiple lines for change-control, and add step >= …

…1 case

specs/phase0/p2p-interface.md

@@ -532,7 +532,10 @@ Response Content:
 )
 ```
 
-Requests beacon blocks in the slot range `[start_slot, start_slot + count * step)`, leading up to the current head block as selected by fork choice. `step` defines the slot increment between blocks. For example, requesting blocks starting at `start_slot` 2 with a step value of 2 would return the blocks at slots [2, 4, 6, …]. In cases where a slot is empty for a given slot number, no block is returned. For example, if slot 4 were empty in the previous example, the returned array would contain [2, 6, …].
+Requests beacon blocks in the slot range `[start_slot, start_slot + count * step)`, leading up to the current head block as selected by fork choice.
+`step` defines the slot increment between blocks. For example, requesting blocks starting at `start_slot` 2 with a step value of 2 would return the blocks at slots [2, 4, 6, …].
+In cases where a slot is empty for a given slot number, no block is returned. For example, if slot 4 were empty in the previous example, the returned array would contain [2, 6, …].
+A request MUST NOT have a 0 slot increment, i.e. `step >= 1`.
 
 `BeaconBlocksByRange` is primarily used to sync historical blocks.