[Edit] C++ functions

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Add new content (definitions, codeblocks, etc.)

Description (optional)

We would like to edit the functions concept entry under C++. The entry lives in content/cpp/concepts/functions/functions.md

Please refer to the concept entry template, content standards and markdown style guide when working on the PR for this issue.

Please:

1. Shorten the description to not more than 160 characters
2. Fix any typo in the entry

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