document and change behavior of doc:find with no arguments

[krader1961]

What new feature should Elvish have?

The doc:find function when run with no arguments outputs all of the symbols (variable and function names) it knows about. This should be documented. It also outputs those symbols with a trailing colon (e.g., os:mkdir:) that should be omitted. It also only outputs symbols in the builtin: namespace without that namespace prefix. It is probably preferable that it output both forms; e.g., all and builtin:all. Whether the latter form should only be included if the user has explicitly done use builtin is an open question but I'm inclined to output both forms regardless of whether use builtin has been executed.

Output of "elvish -version"

0.21.0-dev.0.20240320065707-10fa3d2aaf67

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