Update macOS docs

[dveeden]

Installing DBD::mysql with a Perl version that was installed with brew works fine. See also #402

However the problem with system Perl is this:

ec2-user@ip-172-31-29-229 ~ % sw_vers 
ProductName:    macOS
ProductVersion: 12.6.9
BuildVersion:   21G726
ec2-user@ip-172-31-29-229 ~ % perl -e 'print "find " . qq(@INC) . " -name DBIXS.h\n"'
find /Library/Perl/5.30/darwin-thread-multi-2level /Library/Perl/5.30 /Network/Library/Perl/5.30/darwin-thread-multi-2level /Network/Library/Perl/5.30 /Library/Perl/Updates/5.30.3/darwin-thread-multi-2level /Library/Perl/Updates/5.30.3 /System/Library/Perl/5.30/darwin-thread-multi-2level /System/Library/Perl/5.30 /System/Library/Perl/Extras/5.30/darwin-thread-multi-2level /System/Library/Perl/Extras/5.30 -name DBIXS.h
ec2-user@ip-172-31-29-229 ~ % find /Library/Perl/5.30/darwin-thread-multi-2level /Library/Perl/5.30 /Network/Library/Perl/5.30/darwin-thread-multi-2level /Network/Library/Perl/5.30 /Library/Perl/Updates/5.30.3/darwin-thread-multi-2level /Library/Perl/Updates/5.30.3 /System/Library/Perl/5.30/darwin-thread-multi-2level /System/Library/Perl/5.30 /System/Library/Perl/Extras/5.30/darwin-thread-multi-2level /System/Library/Perl/Extras/5.30 -name DBIXS.h
find: /Network/Library/Perl/5.30/darwin-thread-multi-2level: No such file or directory
find: /Network/Library/Perl/5.30: No such file or directory
ec2-user@ip-172-31-29-229 ~ % find /Library/Developer/CommandLineTools/SDKs -name DBIXS.h
/Library/Developer/CommandLineTools/SDKs/MacOSX13.1.sdk/System/Library/Perl/Extras/5.30/darwin-thread-multi-2level/auto/DBI/DBIXS.h
/Library/Developer/CommandLineTools/SDKs/MacOSX12.3.sdk/System/Library/Perl/Extras/5.30/darwin-thread-multi-2level/auto/DBI/DBIXS.h
/Library/Developer/CommandLineTools/SDKs/MacOSX12.3.sdk/System/Library/Perl/Extras/5.18/darwin-thread-multi-2level/auto/DBI/DBIXS.h

There is no DBIXS.h in @INC. There is one included with the Xcode commandline tools SDKs, but either way using a brew version seems to be more reliable and seems to be recommended by most people.