Remove two no longer relevant problem from the drawbacks section

Remove two no longer relevant problem from the drawbacks section

Output of the first command is now:

$ git branch -D foo
error: Unable to create '.git/packed-refs.lock': File exists.

If no other git process is currently running, this probably means a
git process crashed in this repository earlier. Make sure no other git
process is running and remove the file manually to continue.
error: cannot delete 'refs/heads/foo' from packed refs
error: Error deleting branch 'foo'

The second:

$ git push
(...)
warning: updating the current branch
warning: Updating the currently checked out branch may cause confusion,
warning: as the index and work tree do not reflect changes that are in HEAD.
warning: As a result, you may see the changes you just pushed into it
warning: reverted when you run 'git diff' over there, and you may want
warning: to run 'git reset --hard' before starting to work to recover.
warning:
warning: You can set 'receive.denyCurrentBranch' configuration variable to
warning: 'refuse' in the remote repository to forbid pushing into its
warning: current branch.
warning: To allow pushing into the current branch, you can set it to 'ignore';
warning: but this is not recommended unless you arranged to update its work
warning: tree to match what you pushed in some other way.
warning:
warning: To squelch this message, you can set it to 'warn'.
warning:
warning: Note that the default will change in a future version of git
warning: to refuse updating the current branch unless you have the
warning: configuration variable set to either 'ignore' or 'warn'.