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

Emmm... Maybe I'm splitting hairs. But I really think the paragraph
should be more detailed. The orginal document makes me confused. Do I
take the ownership of the string data?
Here I don't refer the `clang_disposeString` function, because here's a
`clang_disposeStringSet`.

Co-authored-by: Saleem Abdulrasool <[email protected]>

Author: iseki0

clang/include/clang-c/CXString.h

@@ -46,6 +46,10 @@ typedef struct {
 
 /**
  * Retrieve the character data associated with the given string.
+ *
+ * The returned data is a reference and not owned by the user. This data
+ * is only valid while the `CXString` is valid. This function is similar
+ * to `std::string::c_str()`.
  */
 CINDEX_LINKAGE const char *clang_getCString(CXString string);