Merge #18817: doc: Document differences in bitcoind and bitcoin-qt locale handling

ca185cf5a1 doc: Document differences in bitcoind and bitcoin-qt locale handling (practicalswift)

Pull request description:

  Document differences in `bitcoind` and `bitcoin-qt` locale handling.

  Since this seems to be the root cause to the locale dependency issues we've seen over the years I thought it was worth documenting :)

  Note that 1.) `QLocale` (used by Qt), 2.) C locale (used by locale-sensitive C standard library functions/POSIX functions and some parts of the C++ standard library such as `std::to_string`) and 3.) C++ locale (used by the C++ input/output library) are three separate things. This comment is about the perhaps surprising interference with the C locale (2) that takes place as part of the Qt initialization.

ACKs for top commit:
  hebasto:
    re-ACK ca185cf5a1

Tree-SHA512: e51c32f3072c506b0029a001d8b108125e1acb4f2b6a48a6be721ddadda9da0ae77a9b39ff33f9d9eebabe2244c1db09e8502e3e7012d7a5d40d98e96da0dc44

src/qt/bitcoin.cpp

@@ -206,6 +206,7 @@ BitcoinApplication::BitcoinApplication():
     returnValue(0),
     platformStyle(nullptr)
 {
+    // Qt runs setlocale(LC_ALL, "") on initialization.
     RegisterMetaTypes();
     setQuitOnLastWindowClosed(false);
 }