[DOC] Enhanced RDoc for MatchData (#5819)

Treats:
    #begin
    #end
    #match
    #match_length

doc/matchdata/begin.rdoc

@@ -0,0 +1,30 @@
+Returns the offset (in characters) of the beginning of the specified match.
+
+When non-negative integer argument +n+ is given,
+returns the offset of the beginning of the <tt>n</tt>th match:
+
+  m = /(.)(.)(\d+)(\d)/.match("THX1138.")
+  # => #<MatchData "HX1138" 1:"H" 2:"X" 3:"113" 4:"8">
+  m[0]       # => "HX1138"
+  m.begin(0) # => 1
+  m[3]       # => "113"
+  m.begin(3) # => 3
+
+  m = /(т)(е)(с)/.match('тест')
+  # => #<MatchData "тес" 1:"т" 2:"е" 3:"с">
+  m[0]       # => "тес"
+  m.begin(0) # => 0
+  m[3]       # => "с"
+  m.begin(3) # => 2
+
+When string or symbol argument +name+ is given,
+returns the offset of the beginning for the named match:
+
+  m = /(?<foo>.)(.)(?<bar>.)/.match("hoge")
+  # => #<MatchData "hog" foo:"h" bar:"g">
+  m[:foo]        # => "h"
+  m.begin('foo') # => 0
+  m[:bar]        # => "g"
+  m.begin(:bar)  # => 2
+
+Related: MatchData#end, MatchData#offset, MatchData#byteoffset.

doc/matchdata/end.rdoc

@@ -0,0 +1,30 @@
+Returns the offset (in characters) of the end of the specified match.
+
+When non-negative integer argument +n+ is given,
+returns the offset of the end of the <tt>n</tt>th match:
+
+  m = /(.)(.)(\d+)(\d)/.match("THX1138.")
+  # => #<MatchData "HX1138" 1:"H" 2:"X" 3:"113" 4:"8">
+  m[0]     # => "HX1138"
+  m.end(0) # => 7
+  m[3]     # => "113"
+  m.end(3) # => 6
+
+  m = /(т)(е)(с)/.match('тест')
+  # => #<MatchData "тес" 1:"т" 2:"е" 3:"с">
+  m[0]     # => "тес"
+  m.end(0) # => 3
+  m[3]     # => "с"
+  m.end(3) # => 3
+
+When string or symbol argument +name+ is given,
+returns the offset of the end for the named match:
+
+  m = /(?<foo>.)(.)(?<bar>.)/.match("hoge")
+  # => #<MatchData "hog" foo:"h" bar:"g">
+  m[:foo]      # => "h"
+  m.end('foo') # => 1
+  m[:bar]      # => "g"
+  m.end(:bar)  # => 3
+
+Related: MatchData#begin, MatchData#offset, MatchData#byteoffset.

doc/matchdata/offset.rdoc

@@ -29,4 +29,3 @@ returns the starting and ending offsets for the named match:
   m.offset(:bar)  # => [2, 3]
 
 Related: MatchData#byteoffset, MatchData#begin, MatchData#end.
-