[LangRef] callbr result can be used in all successors (#91167)

Originally, the callbr result could only be used on the fallthrough
destination. This limitation has been lifted, and the result is now also
available on the indirect destinations. However, LangRef was not updated
to reflect this.

Author: nikic

llvm/docs/LangRef.rst

@@ -9113,8 +9113,8 @@ instruction in most regards. The primary difference is that it
 establishes an association with additional labels to define where control
 flow goes after the call.
 
-The output values of a '``callbr``' instruction are available only to
-the '``fallthrough``' block, not to any '``indirect``' blocks(s).
+The output values of a '``callbr``' instruction are available both in the
+the '``fallthrough``' block, and any '``indirect``' blocks(s).
 
 The only use of this today is to implement the "goto" feature of gcc inline
 assembly where additional labels can be provided as locations for the inline