Clarify which `rbenv install` flags get forwarded to `ruby-build`

#!/usr/bin/env bash

#

# Summary: Install a Ruby version using the ruby-build plugin

# Summary: Install a Ruby version using ruby-build

#

# Usage: rbenv install [-fkvp] <version>

#        rbenv install [-fkvp] <definition-file>

# Usage: rbenv install [-f] [-kvp] <version>

#        rbenv install [-f] [-kvp] <definition-file>

#        rbenv install -l|--list

#

#   -l/--list        List all available versions

#   -f/--force       Install even if the version appears to be installed already

#

#   ruby-build options:

#

#   -k/--keep        Keep source tree in $RBENV_BUILD_ROOT after installation

#                    (defaults to $RBENV_ROOT/sources)

#   -v/--verbose     Verbose mode: print compilation status to stdout