[lldb][test][NFC] Document DYLIB_NAME Makefile variable (llvm#112735)

Got caught out by this because simply specifying `DYLIB_CXX_SOURCES`
(without specifying `DYLIB_NAME`) resulted in linker errors because the
dylib was never built (and linked). We should probably make that a
Makefile error (though I haven't audited when exactly not specifying
`DYLIB_NAME` is valid; looked like that can happen when we specify
`FRAMEWORK`).

Author: Michael137

lldb/packages/Python/lldbsuite/test/make/Makefile.rules

@@ -13,6 +13,13 @@
 # the building of the a.out executable program.  For example,
 # DYLIB_ONLY := YES
 #
+# When specifying one of the DYLIB_*_SOURCES variables, DYLIB_NAME
+# controls the (platform-dependent) name of the produced dylib. E.g.,
+# on Darwin, if "DYLIB_NAME := foo", the generated dylib will be called
+# "libfoo.dylib".
+#
+# DYLIB_NAME := foo
+#
 # Specifying FRAMEWORK and its variants has the effect of building a NeXT-style
 # framework.
 # FRAMEWORK := "Foo"