code-style: Update QString rule

[ptitjano]

See: qgis/QGIS#64395

cc @rouault

[nyalldawson]

I'm not sure what the right way to proceed with this is. If we follow policy we should do the announce on lists, two week timeout + voting, which definitely seems overkill here. On the other hand I'm very reluctant to start making exceptions to the Qep process, as that's a slippery slope and dangerous precedent to set... 🤔. What's everyone else think?

[rouault]

What's everyone else think?

Given that we have already adopted in the code the new way, this is just a matter of making the doc reflect the "established" practice. Anyway we can follow the rules regarding the update of the QEP. This PR isn't going to rot within 2 weeks.

[3nids]

I generally tend to go for simplicity/clarity, and I would say that the simplicity is to stick to the process. Otherwise, it will take more time to formulate when and how we can make exceptions…
(But if people thinks it's super clear that this one goes through directly, I'm perfectly fine with this!)

[uclaros]

I wouldn't say this is an exception to the QEP process, as this is technically not a QEP, as Even mentioned.
The exception to the process would be the actual qgis/QGIS#64395 getting merged without a QEP, but everyone was fed up with the QStringLiterals AND it was asked on the mailing list, AND it's been more than two weeks since the mailing list post, so while there was not a QEP, all the boxes are now ticked.

I was wondering whether we should document the usage of using namespace Qt::StringLiterals; and where it should be included (adding it in every file? as few files as possible? somewhere in between?)?