Correct key file doc

[pombredanne]

Description

We have this https://github.com/nexB/scancode-toolkit/blob/ebb55c341bc6ba644d2be0e531ffbfc2948bca47/docs/source/cli-reference/scan-options-pre.rst#L75

key-file
A KEY file is a generic file extension used by various programs when registering legal copies of the software. It may be saved in a plain text format, but generally contains some form of encrypted key string that authenticates the purchase and registers the software.

A key file is not a license key file. This is a key more as in "keystone", e.g. an important file for the documentation of a package.
See https://github.com/nexB/scancode-toolkit/blob/ebb55c341bc6ba644d2be0e531ffbfc2948bca47/src/summarycode/classify.py#L74 :

True if this file is "top-level" file and either a legal, readme or manifest file.

We need to fix this description

[Ripoohann]

Hey I want to work on this issue can you please elaborate

[chinyeungli]

@pombredanne / @AyanSinhaMahapatra
I am not sure if this following description correct

key-file
A KEY file serves as a keystone element, containing essential information about a software package—such as its dependencies, versioning, licensing, and more.

Please review and provide any suggested or alternative descriptions.

[AyanSinhaMahapatra]

@chinyeungli this is better. We might want to add another line highlighting how these key-files often contain the primary-license of the package or the overall license of the package, among other package metadata which are general or ecosystem specific. This is best to review/suggest additions to in a PR