docs(man): write about the combination of a kind letter and a kind name

Signed-off-by: Masatake YAMATO <yamato@redhat.com>

man/ctags-incompatibilities.7.rst.in

@@ -128,6 +128,25 @@ includes a whitespace character.
 This requirement is for making the output of Universal-ctags follow
 the tags file format.
 
+A combination of a kind letter and a kind name
+....................................................................................
+
+In Universal-ctags, the combination of
+a kind letter and a kind name must be unique in a language.
+
+You cannot define more than one kinds reusing a kind letter with
+different kind names. You cannot define more than one kinds reusing a
+kind name with different kind letters.
+
+An example::
+
+  --regex-Foo=/abstract +class +([a-z]+)/\1/a,abstractClass/i
+  --regex-Foo=/attribute +([a-z]+)/\1/a,attribute/i
+
+Universal-ctags rejects this because the kind letter, "a", used twice
+for defining a kind "abstractClass" and "attribute".
+
+
 Incompatibilities in tags file format
 -------------------------------------------------------------