Fix various lint warnings in manuals

Fix "new sentence, new line" warnings throughout so that formatters
can produce correct spacing between sentences.

join.1
  Remove unnecessary Ns macros. These are not necessary for delimeters,
  which get special treatment.

xinstall.1
  Fix date in manual. The contents were last modified on 2016-12-03,
  so use that instead of the invalid date.

grep.1
  Fix escape sequence for `\<` and `\>`.

ed.1
  Remove spurious `\\n` escape for the null-command.

expr.1

@@ -22,7 +22,8 @@ Let
 be a non-elemental expression and
 .Sy expr1 ,
 .Sy expr2
-arbitrary expressions. Then
+arbitrary expressions.
+Then
 .Sy expr
 has the recursive form
 .Sy expr = [(] expr1 operand expr2 [)].
@@ -76,8 +77,9 @@ against
 is anchored with an implicit '^'.
 .Pp
 You can't directly match the empty string, since zero matched characters
-resolve equally to a failed match. To work around this limitation, use
-"expr X'' : 'X$' instead of "expr '' : '$'"
+resolve equally to a failed match.
+To work around this limitation, use "expr X'' : 'X$' instead of "expr ''
+: '$'"
 .El
 .Sh EXIT STATUS
 .Bl -tag -width Ds