docs: document table default for multi-value options in cli:option docstring

[Copilot]

The cli:option default parameter docstring typed the argument as {bool} and only described scalar defaults — silently omitting the table form that enables multi-value collection. This is the primary place users and generated API docs reference, making the feature effectively invisible there despite being in the README and test suite.

Changes

• src/cliargs/core.lua — cli:option @param default:
  • Type annotation: {bool} → {string|table}
  • Added description of table behaviour: passing a table makes the option accumulate values across invocations; pre-populated entries are the default list

-- Passing a table enables multi-value collection:
cli:option('--tag=VALUE', 'one or more tags', {})
-- cli --tag foo --tag bar  →  args["tag"] == { "foo", "bar" }

-- Pre-populated entries act as defaults when the option is never supplied:
cli:option('--compress=VALUE', 'compression algorithm', { 'lzma' })
-- cli  →  args["compress"] == { "lzma" }

fixes #70

[Tieske]

lgtm