Merge bitcoin#32017: doc: warn against having qt6 installed on macOS

d79dab0 doc: warn against having qt6 installed on macOS (Sjors Provoost)

Pull request description:

  Document bitcoin#31009 in time for the v29 release.

ACKs for top commit:
  achow101:
    ACK d79dab0
  hebasto:
    ACK d79dab0.

Tree-SHA512: 4c6e557b6410c7fd766e1cdc356ae9f7410fbb4746732580e5bdf33ba43dca64e6f2fb66677d1e0c8fa71c19f212dc81ac73dc4277f2fd966bbd41c20d9291f8

Author: hebasto

doc/build-osx.md

@@ -95,6 +95,8 @@ Qt, libqrencode and pass `-DBUILD_GUI=ON`. Skip if you don't intend to use the G
 brew install qt@5
 ```
 
+Note: Building may fail if Qt 6 is installed (`qt` or `qt@6`)
+
 Note: Building with Qt binaries downloaded from the Qt website is not officially supported.
 See the notes in [#7714](https://github.com/bitcoin/bitcoin/issues/7714).