Add some documentetion to tsetmode

Add some documentetion to tsetmode

The names of the terminal modes supported by vt102 are (taken from the VT220
programmer reference manual):

Table 4-7 ANSI-Standardized Modes
Name  Mnemonic		    Parameter (Ps)
Error (ignored)		    -         0 (3/0)
Keyboard action		    KAM	      2 (3/2)
Insert/replace		    IRM	      4 (3/4)
Send/receive		    SRM	      12 (3/1 3/2)
Line feed/new line	    LNM       20 (3/2 3/0)

Table 4-8 ANSI-Compatible DEC Private Modes
Name  Mnemonic		  Parameter (Ps)
Error (ignored)		  -	    0 (3/0)
Cursor key		  DECCKM    1 (3/1)
ANSI/VT52		  DECANM    2 (3/2)
Column			  DECCOLM   3 (3/3)
Scroll			  DECSCLM   4 (3/4)
Screen			  DECSCNM   5 (3/5)
Origin			  DECOM	    6 (3/6)
Auto wrap		  DECAWM    7 (3/7)
Auto repeat		  DECARM    8 (3/8)
Printer form feed	  DECPFF    18 (3/1 3/8)
Printer extent		  DECPEX    19 (3/1 3/9)
Text cursor enable	  DECTCEM   25 (3/2 3/5)
National replacement character sets DECNRCM 42 (3/4 3/2)

This patch adds a comment for each one of these sequences.
---
 st.c |   26 +++++++++++++++++++++-----
 1 file changed, 21 insertions(+), 5 deletions(-)