doc: document convention of placing `&` and `const`

[nfeske]

While working on #5517 throughout the code base, a few coding-style inconsistencies caught my eye. I noticed that our coding-style documentation actually does not speak about our predominant practices of placing reference (&) and pointer (*) characters at variable declarations. Also the placement of qualifiers like const is not covered. To foster consistency, we should close these documentation gaps.

[nfeske]

Commit 5268de0 adds the missing documentation.

Commit eaf9058 applies the convention.