[clang] [docs] Add a more detailed description in CXString.h. (PR #119090)

[via cfe-commits]

================
@@ -46,6 +46,9 @@ typedef struct {
 
 /**
  * Retrieve the character data associated with the given string.
+ *
+ * The caller shouldn't free the returned string data, and the returned string
+ * data shouldn't be accessed after the \c CXString disposed.
----------------
iseki0 wrote:

Sounds great, but in your sentence, you may have to define what is "valid". (You said "while the `CXString` is valid.") 
@compnerd 

https://github.com/llvm/llvm-project/pull/119090