Update the reason llvm tracing is disabled by default

Co-authored-by: Andreas Jonson <[email protected]>

Author: cuviper

src/doc/unstable-book/src/compiler-flags/self-profile-events.md

@@ -41,7 +41,7 @@ $ rustc -Zself-profile -Zself-profile-events=default,args
 
 - `llvm`
   - Adds tracing information about LLVM passes and codegeneration.
-  - Disabled by default for historical reasons.
+  - Disabled by default because this significantly increases the trace file size.
 
 ## Event synonyms