bitbake-metadata2doc.py: rename functions after the filenames they write

Signed-off-by: Mario Domenech Goulart <mario@ossystems.com.br>
release-notes/source/kernel-linux.rst: fix typos; some rewording

Signed-off-by: Mario Domenech Goulart <mario@ossystems.com.br>
bitbake-metadata2doc.py: use dashes instead of bullets to indicate "no version"

Signed-off-by: Mario Domenech Goulart <mario@ossystems.com.br>
bitbake-metadata2doc.py: append branch information to versions, if it exists

Signed-off-by: Mario Domenech Goulart <mario@ossystems.com.br>