mandoc: Document the "delim" syntax and its usage.

[schwarze@mandoc.bsd.lv]

Log Message:
-----------
Document the "delim" syntax and its usage.
Closing a gap reported by bentley@, who also sent a patch,
but i'm explaining it somewhat differently.
While here, remove duplicate information from the text.
OK bentley@

Modified Files:
--------------
    mandoc:
        eqn.7

Revision Data
-------------
Index: eqn.7
===================================================================
RCS file: /home/cvs/mandoc/mandoc/eqn.7,v
retrieving revision 1.38
retrieving revision 1.39
diff -Leqn.7 -Leqn.7 -u -p -r1.38 -r1.39
--- eqn.7
+++ eqn.7
@@ -44,28 +44,16 @@ specification (see
 .Sx SEE ALSO
 for references).
 .Pp
-Equations within
-.Xr mdoc 7
-or
-.Xr man 7
-documents are enclosed by the standalone
-.Sq \&.EQ
-and
-.Sq \&.EN
-tags.
-Equations are multi-line blocks consisting of formulas and control
-statements.
-.Sh EQUATION STRUCTURE
-Each equation is bracketed by
-.Sq \&.EQ
-and
-.Sq \&.EN
-strings.
-.Em Note :
-these are not the same as
-.Xr roff 7
-macros, and may only be invoked as
-.Sq \&.EQ .
+An equation starts with an input line containing exactly the characters
+.Sq \&.EQ ,
+may contain multiple input lines, and ends with an input line
+containing exactly the characters
+.Sq \&.EN .
+Equivalently, an equation can be given in the middle of a single
+text input line by surrounding it with the equation delimiters
+defined with the
+.Cm delim
+statement.
 .Pp
 The equation grammar is as follows, where quoted strings are
 case-sensitive literals in the input:
@@ -178,6 +166,25 @@ statement is a synonym for
 while
 .Cm tdefine
 is discarded.
+.It Cm delim
+This statement takes a string argument consisting of two bytes,
+to be used as the opening and closing delimiters for equations
+in the middle of text input lines.
+Conventionally, the dollar sign is used for both delimiters,
+as follows:
+.Bd -literal -offset indent
+\&.EQ
+delim $$
+\&.EN
+An equation like $sin pi = 0$ can now be entered
+in the middle of a text input line.
+.Ed
+.Pp
+The special statement
+.Cm delim off
+temporarily disables previously declared delimiters and
+.Cm delim on
+reenables them.
 .It Cm gfont
 Set the default font of subsequent output.
 Its syntax is as follows:
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