[Clang][doc] Fix __builtin_assume wording.

[Mordante]

Summary

D117296 removed wording for __builtin_assume, D120205 restored the
wording, but the last sentence was only partly restored. This restores
the rest of the last sentence.

[aaron.ballman]

LGTM, a second time. :-D Sorry for missing that the first time.

[Mordante]

In D122423#3406381, @aaron.ballman wrote:

LGTM, a second time. :-D Sorry for missing that the first time.

No problem, thanks for the review.

clang/docs/LanguageExtensions.rst

int foo(int x) {
       return do_something_else();
   }
 
 **Description**:
 
 The boolean argument to this function is defined to be true. The optimizer may
 analyze the form of the expression provided as the argument and deduce from
 that information used to optimize the program. If the condition is violated
-during execution, the behavior is undefined. The argument itself is 
+during execution, the behavior is undefined. The argument itself is never
+evaluated, so any side effects of the expression will be discarded.
 
 Query for this feature with ``__has_builtin(__builtin_assume)``.
 
 ``__builtin_call_with_static_chain``
 ------------------------------------
 
 ``__builtin_call_with_static_chain`` is used to perform a static call while
 setting updating the static chain register.