version-file docs update

[richiethomas]

The job of the rbenv-version-file command is to print the filepath for the file which sets the current Ruby version. It searches for a .ruby-version file, traversing upward if such a file is not found until it reaches the root directory.

• If a directory is passed as an argument, the command uses that directory as a starting point.
• If no directory is passed as an argument:
  • The command first searches in RBENV_DIR.
  • If no .ruby-version file is found, it searches again in the current directory (provided that RBENV_DIR is different from the current directory).
  • If both of those attempts fail to find a .ruby-version file, it prints the path to RBENV's global version file ("${RBENV_ROOT}/version").

This global version file may not actually exist, for example in the following cases:

• If a user has not yet set a global version via rbenv global <version_number>.
• If they have not set a local project version, either via rbenv local <version_number> or by manually creating a .ruby-version file.

In the case where the filepath returned by rbenv-version-file doesn't actually exist, the result might be confusion on the user's part. This PR attempts to address that confusion, by updating the output of rbenv help version-file to specify that this command's output is the filepath which would set the Ruby version, if it exists.

[mislav]

Minor: the summaries of other commands don't end in periods, so this one shouldn't either.

[mislav]

Thank you! I have pushed amendments to the text for more clarity.