README: Split up documentation of table extensions.

README

@@ -1455,15 +1455,14 @@ A line containing a row of three or more `*`, `-`, or `_` characters
 Tables
 ------
 
-**Extension: `simple_tables`, `multiline_tables`, `grid_tables`,
-`pipe_tables`, `table_captions`**
-
 Four kinds of tables may be used. The first three kinds presuppose the use of
 a fixed-width font, such as Courier. The fourth kind can be used with
 proportionally spaced fonts, as it does not require lining up columns.
 
 ### Simple tables
 
+**Extension: `simple_tables`, `table_captions`**
+
 Simple tables look like this:
 
       Right     Left     Center     Default
@@ -1511,6 +1510,8 @@ would be right, left, center, and right aligned, respectively.
 
 ### Multiline tables
 
+**Extension: `multiline_tables`, `table_captions`**
+
 Multiline tables allow headers and table rows to span multiple lines
 of text (but cells that span multiple columns or rows of the table are
 not supported).  Here is an example:
@@ -1561,6 +1562,8 @@ the table), or the table may be interpreted as a simple table.
 
 ### Grid tables
 
+**Extension: `grid_tables`, `table_captions`**
+
 Grid tables look like this:
 
     : Sample grid table.
@@ -1585,6 +1588,8 @@ columns or rows. Grid tables can be created easily using [Emacs table mode].
 
 ### Pipe tables
 
+**Extension: `pipe_tables`, `table_captions`**
+
 Pipe tables look like this:
 
     | Right | Left | Default | Center |