docs: add note for custom_target character replacement

[kimburgess]

Added note about implicit backslash character replacement within all custom_target command arguments.

meson/mesonbuild/backend/backends.py

Lines 1583 to 1601 in 64e3f46

	# This should not be necessary but removing it breaks
	# building GStreamer on Windows. The underlying issue
	# is problems with quoting backslashes on Windows
	# which is the seventh circle of hell. The downside is
	# that this breaks custom targets whose command lines
	# have backslashes. If you try to fix this be sure to
	# check that it does not break GST.
	#
	# The bug causes file paths such as c:\foo to get escaped
	# into c:\\foo.
	#
	# Unfortunately we have not been able to come up with an
	# isolated test case for this so unless you manage to come up
	# with one, the only way is to test the building with Gst's
	# setup. Note this in your MR or ping us and we will get it
	# fixed.
	#
	# https://github.com/mesonbuild/meson/pull/737
	cmd = [i.replace('\\', '/') if isinstance(i, str) else i for i in cmd]

This behaviour leads to some extremely confusing and hard to debug build issues. The root issue (#1564) appears to be long running and has some contention around previous attempts to resolve. While this remains active it seems sensible to make side effects of the current workaround clear.