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.

date.1

@@ -41,8 +41,8 @@ sign signals a user-defined format string using
 conversion specifications.
 .Pp
 An operand without a leading plus sign is interpreted as a value
-for setting the system's current date and time. The canonical
-representation for setting the date and time is:
+for setting the system's current date and time.
+The canonical representation for setting the date and time is:
 .Pp
 .Bl -tag -width Ds -compact -offset indent
 .It Ar mm
@@ -65,12 +65,14 @@ is not, a value for
 .Ar yy
 between 69 and 99 results in a
 .Ar CC
-value of 19. Otherwise, a
+value of 19.
+Otherwise, a
 .Ar CC
 value of 20 is used.
 .El
 .Pp
-The century and year are optional. The default is the current year.
+The century and year are optional.
+The default is the current year.
 .Sh STANDARDS
 The
 .Nm