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

[Saleem Abdulrasool via cfe-commits]

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@@ -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.
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compnerd wrote:

Whoops! That's my mistake - I meant to say `not`, not `now`. The data is the inner pointer like `c_str` is, and not meant to be modified by the user.

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