man pages: add missing commas between subordinate and main clauses

jmc@ dislikes a comma before "then" in a conditional, so leave those
untouched.

ok jmc@

Author: naddy

tmux.1

@@ -43,7 +43,7 @@ then later reattached.
 .Pp
 When
 .Nm
-is started it creates a new
+is started, it creates a new
 .Em session
 with a single
 .Em window
@@ -5918,7 +5918,7 @@ does not surround the popup by a border.
 sets the type of border line for the popup.
 When
 .Fl B
-is specified the
+is specified, the
 .Fl b
 option is ignored.
 See
@@ -6639,7 +6639,7 @@ are replaced with underscores
 For input,
 .Nm
 always runs with a UTF-8 locale.
-If en_US.UTF-8 is provided by the operating system it is used and
+If en_US.UTF-8 is provided by the operating system, it is used and
 .Ev LC_CTYPE
 is ignored for input.
 Otherwise,