NL.8: elaborate on reserved identifiers (#2131)

* NL.8: elaborate on reserved identifiers

* Update CppCoreGuidelines.md

* Update CppCoreGuidelines.md

* Update CppCoreGuidelines.md

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Co-authored-by: Herb Sutter <herb.sutter@gmail.com>

CppCoreGuidelines.md

@@ -21626,7 +21626,8 @@ ISO Standard, use lower case only and digits, separate words with underscores:
 * `vector`
 * `my_map`
 
-Avoid double underscores `__`.
+Avoid identifier names that contain double underscores `__` or that start with an underscore followed by a capital letter (e.g., `_Throws`).
+Such identifiers are reserved for the C++ implementation.
 
 ##### Example