doc: Unify explanations of one-manager-operation-per-manager

The following discussion from !3996 (merged) should be addressed:

• @NicVolanschi started a discussion: (+2 comments)

  This paragraph is probably misplaced here, because it not only concerns the precheck filter, but also the prefilter and postfilter (if the explanation in section "One operation manager per manager per block" in ./plugins is true!?).

  So we'd better move it to the beginning of that section, which also improves understanding, because:

  • the feature will be justified there before being explained
  • the logical flow of the current page will be more continuous, focusing on solvability

  If the move is done, mind:

  1. changing the start of the first paragraph in that section (which will become the second paragraph then), from "The mempool filters, ... " to "For this reason, the mempool filters, ...".
  2. changing the first phrase of the present paragraph, to adapt it to its new, broader context, from "However, restricting the propagation to solvable operations is not enough to ensure resilience against DDoS attacks." to "However, filtering manager operations individually is not enough to avoid flooding the network with invalid operations."