Clarify in release docs NOT to use github tagging.

[galak]

If we use github to tag it doesn't create an 'annotated' tag for a pre-release (not sure if it does for a release).

So we get:

[galak@spiff zephyr]$ git describe 
v1.14.0-rc1-1540-g12bf7e6ead

Where git describe --tag does:

[galak@spiff zephyr]$ git describe  --tags
v1.14.0-rc2-302-g12bf7e6ead

[galak]

Not sure if we want to add --tag to 'git describe' or try and make sure we always annotate tags?

[galak]

We should handle this via tagging by hand and not via github. Want to update https://docs.zephyrproject.org/latest/development_process/release_process.html#tagging

to clarify NOT to use github.