Combine duplicate occurrences of an exception

[rpetrusha]

Combine duplicate occurrences of an exception

When a member throws an exception of a particular type, that exception type should appear only once in the documentation for that particular member. Each exception condition should appear as a separate item. The documentation for an exception with multiple exception conditions should look something like the following:

<exception cref="T:System.ArgumentOutOfRangeException">
<paramref name="year" /> is not in the range supported by <paramref name="calendar" />.  
 -or-  
<paramref name="month" /> is less than 1 or greater than the number of months in <paramref name="calendar" />.  
-or-  
<paramref name="day" /> is less than 1 or greater than the number of days in <paramref name="month" />.
</exception>

In some cases, there are multiple <exception> elements for a single exception type. For example:

<exception cref="T:System.ArgumentOutOfRangeException">
   <paramref name="year" /> is not in the range supported by <paramref name="calendar" />.  
</exception>
<exception cref="T:System.ArgumentOutOfRangeException">  
   <paramref name="month" /> is less than 1 or greater than the number of months in <paramref name="calendar" />.  
</exception>

Duplicate exception entries should be identifies and replaced with a single exception entry with multiple exception conditions.

[mairaw]

In the cases where the multiple elements existed, the extra ones were dropped during migration. I've been bringing the ones I'm finding while testing IntelliSense, but it would be good to have a script to take care of this.

/cc @wh-alice @TianqiZhang @yishengjin1413