re.c: Fix conflicting flags for RMatch

`RMATCH_OFFSETS_EXTERNAL` and `MATCH_BUSY` were both assigned to
`FL_USER2`, which could have created weird bugs.

Also add flag documentation for both RMatch and RRegexp.

Author: byroot

re.c

@@ -32,6 +32,30 @@
 #include "ruby/util.h"
 #include "ractor_core.h"
 
+/* Flags of RRegexp
+ *
+ * 4:     KCODE_FIXED
+ *            The regexp has "fixed encoding", meaning it can't be match against any ASCII-compatible string.
+ * 6:     REG_ENCODING_NONE
+ *            The regexp has no encoding. Means the `n` modifier was used.
+ */
+
+#define KCODE_FIXED FL_USER4
+#define REG_ENCODING_NONE FL_USER6
+
+/* Flags of RMatch
+ *
+ * 0:     MATCH_BUSY
+ *            The match is currently in use or may have escaped and can no longer be recycled.
+ * 1:     RMATCH_ONIG
+ *            TBD.
+ * 2:     RMATCH_OFFSETS_EXTERNAL
+ *            The match layout isn't fully embedded, offsets are stored in an external buffer,
+ *            which will need to be freed during sweep.
+ */
+
+#define MATCH_BUSY FL_USER0
+
 VALUE rb_eRegexpError, rb_eRegexpTimeoutError;
 
 typedef char onig_errmsg_buffer[ONIG_MAX_ERROR_MESSAGE_LEN];
@@ -285,10 +309,6 @@ rb_memsearch(const void *x0, long m, const void *y0, long n, rb_encoding *enc)
     return rb_memsearch_qs(x0, m, y0, n);
 }
 
-#define REG_ENCODING_NONE FL_USER6
-
-#define KCODE_FIXED FL_USER4
-
 static int
 char_to_option(int c)
 {
@@ -1528,8 +1548,6 @@ match_nth_length(VALUE match, VALUE n)
     return LONG2NUM(ofs->end - ofs->beg);
 }
 
-#define MATCH_BUSY FL_USER2
-
 void
 rb_match_busy(VALUE match)
 {