Improve warning message

https://github.com/ruby/ruby/pull/2637#discussion_r341812475

re.c

@@ -3324,7 +3324,7 @@ rb_reg_match_m(int argc, VALUE *argv, VALUE re)
     }
 
     if (NIL_P(str)) {
-        rb_warn("given argument is nil");
+        rb_warn("given argument is nil; this will raise a TypeError in the next release");
     }
     pos = reg_match_pos(re, &str, pos);
     if (pos < 0) {
@@ -3372,7 +3372,7 @@ rb_reg_match_p(VALUE re, VALUE str, long pos)
     int tmpreg;
 
     if (NIL_P(str)) {
-        rb_warn("given argument is nil");
+        rb_warn("given argument is nil; this will raise a TypeError in the next release");
         return Qfalse;
     }
     str = SYMBOL_P(str) ? rb_sym2str(str) : StringValue(str);

spec/ruby/core/regexp/match_spec.rb

@@ -100,7 +100,7 @@ describe "Regexp#match" do
 
   ruby_version_is "2.7"..."3.0" do
     it "warns the deprecation when the given argument is nil" do
-      -> { /foo/.match(nil) }.should complain(/given argument is nil/)
+      -> { /foo/.match(nil) }.should complain(/given argument is nil; this will raise a TypeError in the next release/)
     end
   end
 
@@ -151,7 +151,7 @@ describe "Regexp#match?" do
 
   ruby_version_is "2.7"..."3.0" do
     it "warns the deprecation" do
-      -> { /./.match?(nil) }.should complain(/given argument is nil/)
+      -> { /./.match?(nil) }.should complain(/given argument is nil; this will raise a TypeError in the next release/)
     end
   end