[chinese/cpp] a brief addition

At begining, add "In C, character literals do not exist as a distinct type — they are effectively aliases for integers. In C++, however, true character literals of type char were introduced."
The original document only described the difference in size, which was confusing.

Use the [chinese/cpp] tag to locate the relevant Chinese C++ markdown document in the repo, then find the character-literal section. Add the exact sentence at the beginning of that explanatory part and make sure the wording around C vs C++ size/type differences remains coherent. No tests are mentioned; verification is a documentation review/diff check that the new opening note is present and readable.

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