Twenty Fifteen: Inline docs for header.php (Site-Branding class)

[NikV]

Making my initial commit and documenting the site-branding class in the header.php in Twenty Fifteen

[iamtakashi]

@NikV, please make your diff from the root.

[NikV]

Replying to iamtakashi:

@NikV, please make your diff from the root.

Alright, updated file from root.

[iandstewart]

In 29928:

Twenty Fifteen: document the site-branding class in header.php.

Props NikV, fixes #29961.