mandoc: Mention that the .Dd "date" argument is the date of the last

[schwarze@mandoc.bsd.lv]

Log Message:
-----------
Mention that the .Dd "date" argument is the date of the last change.
Triggered by a question from Jason A. Donenfeld.

While here, delete three COMPATIBILITY entries that i fixed some time ago.

Modified Files:
--------------
    mandoc:
        mdoc.7

Revision Data
-------------
Index: mdoc.7
===================================================================
RCS file: /home/cvs/mandoc/mandoc/mdoc.7,v
retrieving revision 1.280
retrieving revision 1.281
diff -Lmdoc.7 -Lmdoc.7 -u -p -r1.280 -r1.281
--- mdoc.7
+++ mdoc.7
@@ -1203,7 +1203,8 @@ Close a
 block.
 Does not have any tail arguments.
 .It Ic \&Dd Cm $\&Mdocdate$ | Ar month day , year
-Document date for display in the page footer.
+Document date for display in the page footer,
+by convention the date of the last change.
 This is the mandatory first macro of any
 .Nm
 manual.
@@ -3084,26 +3085,12 @@ The following problematic behaviour is f
 .Pp
 .Bl -dash -compact
 .It
-.Ic \&Dd
-with non-standard arguments behaves very strangely.
-When there are three arguments, they are printed verbatim.
-Any other number of arguments is replaced by the current date,
-but without any arguments the string
-.Dq Epoch
-is printed.
-.It
-.Ic \&Lk
-only accepts a single link-name argument; the remainder is misformatted.
-.It
 .Ic \&Pa
 does not format its arguments when used in the FILES section under
 certain list types.
 .It
 .Ic \&Ta
 can only be called by other macros, but not at the beginning of a line.
-.It
-.Ic \&%C
-is not implemented (up to and including groff-1.22.2).
 .It
 .Sq \ef
 .Pq font face
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