docs: consolidate docs and help for package spec (#5048)

Many of our commands parse their args via
[npm-package-arg](https://npm.im/npm-package-arg), which is a good
standard way of parsing a "package" argument.  However the docs
surrounding these args are not very consistent.  This can lead to
confusion in commands such as `npm publish` where the behavior is
slightly different than in the past due to this.

This adds a new help command `npm help package-spec` that describes what
this argument is, and can be, and also updates all the commands that
interpret their args this with to refer to them as `<package-spec>`.  It
also adds a link to the new help page on their docs pages.

Author: wraithgar

docs/content/commands/npm-cache.md

@@ -11,11 +11,7 @@ description: Manipulates packages cache
 <!-- see lib/commands/cache.js -->
 
 ```bash
-npm cache add <tarball file>
-npm cache add <folder>
-npm cache add <tarball url>
-npm cache add <git url>
-npm cache add <name>@<version>
+npm cache add <package-spec>
 npm cache clean [<key>]
 npm cache ls [<name>@<version>]
 npm cache verify
@@ -101,6 +97,7 @@ cache`](/commands/npm-cache)
 
 ### See Also
 
+* [package spec](/using-npm/package-spec)
 * [npm folders](/configuring-npm/folders)
 * [npm config](/commands/npm-config)
 * [npmrc](/configuring-npm/npmrc)

docs/content/commands/npm-ci.md

@@ -133,7 +133,7 @@ will *not* run any pre- or post-scripts.
 * Type: null or String
 
 The shell to use for scripts run with the `npm exec`, `npm run` and `npm
-init <pkg>` commands.
+init <package-spec>` commands.
 
 <!-- automatically generated, do not edit manually -->
 <!-- see lib/utils/config/definitions.js -->