Clarify @brief tag as description/title on documentation guide

[davidwendt]

Description

Simple clarification that the @brief doxygen tag in the C++ documentation does not require a period unless the accompanying text is actually a sentence. This helps keep the documentation consistent across the repo.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[davidwendt]

/merge