intro.1: Various improvements

- Add ref. to other common shells dash(1), ksh(1)
- Add a ref. to stdout(3)
- Separate cp and mv descriptions
- Add examples of special cases of cd
- Add su(1) and shutdown(8) ref. for section Logout and poweroff
- Move Control-D to section Logout and poweroff
- Fix some little format errors

Signed-off-by: Stéphane Aulery <saulery@free.fr>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man1/intro.1

@@ -60,12 +60,10 @@ That is faster and more powerful,
 but requires finding out what the commands are.
 Below a bare minimum, to get started.
 .SS Login
-In order to start working, you probably first have to login,
+In order to start working, you probably first have to open a session by
-that is, give your username and password.
+giving, your username and password.
-See also
-.BR login (1).
 The program
-.I login
+.BR login (1)
 now starts a
 .I shell
 (command interpreter) for you.
@@ -85,12 +83,14 @@ The standard one is called
 See also
 .BR ash (1),
 .BR bash (1),
+.BR chsh (1),
 .BR csh (1),
-.BR zsh (1),
+.BR dash (1),
-.BR chsh (1).
+.BR ksh (1),
+.BR zsh (1).
+.LP
+A session might go like:
 .LP
-A session might go like
-
 .RS
 .nf
 .RB "knuth login: " aeb
@@ -134,7 +134,7 @@ maja    0501\-1136285
 $
 .fi
 .RE
-and here typing Control-D ended the session.
+.LP
 The
 .B $
 here was the command prompt\(emit is the shell's way of indicating
@@ -171,15 +171,19 @@ The command
 .I cat
 will show the contents of a file.
 (The name is from "concatenate and print": all files given as
-parameters are concatenated and sent to "standard output", here
+parameters are concatenated and sent to "standard output"
+(see
+.BR stdout (3)),
+here
 the terminal screen.)
 .LP
 The command
 .I cp
 (from "copy") will copy a file.
-On the other hand, the command
+.LP
+The command
 .I mv
-(from "move") only renames it.
+(from "move"), on the other hand, only renames it.
 .LP
 The command
 .I diff
@@ -223,7 +227,14 @@ prints the current directory.
 The command
 .I cd
 changes the current directory.
-Try "cd /" and "pwd" and "cd" and "pwd".
+.LP
+Try alternatively
+.I cd
+and
+.I pwd
+commands and explore
+.I cd
+usage: "cd", "cd .", "cd ..", "cd /" and "cd ~".
 .SS Directories
 The command
 .I mkdir
@@ -315,11 +326,16 @@ and use a browser if you find HTML files there.
 .BR bash (1),
 .BR chsh (1),
 .BR csh (1),
+.BR dash (1),
+.BR ksh (1),
 .BR locate (1),
 .BR login (1),
 .BR man (1),
+.BR su (1),
 .BR xterm (1),
 .BR zsh(1),
 .BR wait (2),
+.BR stdout (3),
 .BR man-pages (7),
-.BR standards (7)
+.BR standards (7),
+.BR shutdown (8)