[xml] xmlStrEqual() documentation

[oliverst online de]

xmlStrEqual() has a pretty bad sentence as documentation:

"Check if both string are equal of have same content Should be a bit more readable and faster than 
xmlStrEqual()"

There are a few errors in it:
- "string" should be "strings"
- the two senetnce should be seperated with a dot
- xmlStrEqual() should be fester then xmlStrEqual()!?