Doc: clarify documentation about `PRESERVED_CYCLES` in [The storage layer] section

Here is the issue after our discussion last week @germanD about the confusion on PRESERVED_CYCLES.

https://tezos.gitlab.io/shell/storage.html#context-component (Store section, 4th paragraph)

For instance, if the history mode is Archive, every block is preserved, with all its metadata. If it is Full with 5 additional cycles, all the cemented cycles will be present but only the 10 most recent cemented cycles will have some metadata kept (that is, 5 + 5 = 10 as the PRESERVED_CYCLES protocol parameter, which on Mainnet is currently set to 5 cycles, forces the node to keep at least these cycles).

It could be nice to clarify that there is 5 cycles by default on any network (If I'm not mistaken).

... and according to the network on which you are there is an additional 5 (on Mainnet) or 1 cycle (on Mondaynet/Dailynet) but this is already kind of explained at the end of the paragraph.