chore: document canEnterTransactionPool()

[vasild]

Explain that if canEnterTransactionPool() returns false it would have
called processor.pushError().

In addition:

Simplify some code that did "if (A) { return false } return true"
to "return !A".

Remove redundant processor.pushError() from
DelegateResignationTransactionHandler() because
typeFromSenderAlreadyInPool() would have already pushed a very similar
error message.

Closes #3322

[codecov]

Codecov Report

Merging #3323 into develop will increase coverage by 0.05%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##           develop    #3323      +/-   ##
===========================================
+ Coverage     65.9%   65.96%   +0.05%     
===========================================
  Files          431      431              
  Lines        12193    12183      -10     
  Branches      1640     1636       -4     
===========================================
  Hits          8036     8036              
+ Misses        4124     4114      -10     
  Partials        33       33

Impacted Files	Coverage Δ
...core-transactions/src/handlers/second-signature.ts	68% <0%> (+5.03%)	⬆️
packages/core-transactions/src/handlers/vote.ts	65.51% <0%> (+2.18%)	⬆️
...-transactions/src/handlers/delegate-resignation.ts	76.19% <0%> (+6.62%)	⬆️
.../core-transactions/src/handlers/multi-signature.ts	68.62% <0%> (+2.58%)	⬆️

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