add desc

tex-and-texbook/ch6desc.md

@@ -5,7 +5,7 @@
 
 可以指定頁面的一行水平寬度為5inch，再匯入TeX Ch6 範例檔案story的內容，當然最後要加入\end，原檔案沒有\end。
 
-如果設定\hsize太小會出現警告訊息：
+如果設定\hsize太小會出現警告訊息，因為水平盒子無法把東西安放到固定寬度的範圍，所以出現overfull警告和句尾的黑色實心方框：
 ```
 Overfull \hbox (1.34123pt too wide) in paragraph at lines 7--11
 []\tenrm Once upon a|
@@ -32,4 +32,13 @@ Overfull \hbox (7.56346pt too wide) in paragraph at lines 12--16
 \tenrm was hap-pi-est when|
 
 Overfull \hbox (15.03568pt too wide) in paragraph at lines 12--16
-\tenrm he was at work type-|```
+\tenrm he was at work type-|```
+
+
+```\hyphenation{gal-axy}```
+
+強制指定自定連字號劃設規則。這是primitive指令。
+
+```Underfull \hbox (badness 1521) in paragraph at lines 7--11```
+\tolerance=1600
+設定換行允許的badness（不平整度，比如太密或太疏）程度