Extend and improve documentation on subcommands (#890)

[deining]

In an order to eventually close #890, I authored this PR, I hope you like it.
Feel free to perform some polishment like you did before.

[codecov-commenter]

Codecov Report

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@@            Coverage Diff            @@
##             master    #1187   +/-   ##
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  Coverage          ?   94.30%           
  Complexity        ?      455           
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  Files             ?        2           
  Lines             ?     6673           
  Branches          ?     1793           
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  Hits              ?     6293           
  Misses            ?      102           
  Partials          ?      278           

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/picocli/AutoComplete.java	97.01% <0.00%> (ø)	141.00% <0.00%> (?%)
src/main/java/picocli/CommandLine.java	94.14% <0.00%> (ø)	314.00% <0.00%> (?%)

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[remkop]

Thank you for the contribution again!
I plan to merge this soon, I have some other commitments I need to take care of first.

[remkop]

Merged. Thank you for the contribution!
I am also rewriting and rearranging some of the other subsections of the Subcommands section.

About the new Quick Guide example:

The ISOCodeResolver application has two subcommands, language and country, that may be used to either resolve either languages or country codes following the ISO standard.

Can you clarify (ideally with links) which ISO standard(s) this refers to?

[remkop]

Ah sorry, I found that the code example references the exact ISOs. Got it.