Add experimental tag

[jiasli]

The experimental tag (same as preview) appears in following locations:

Command group

Help of parent command group: [Tag]

> python examples\exapp2 -h

Subgroups:
    exp [Experimental] : An experimental command group.

Help of itself: message

> python examples\exapp2 exp -h

Group
    exapp2 exp : An experimental command group.
        This command group is experimental and not covered by customer support. Please use with
        discretion.

Command

Help of parent command group: [Tag]

> python examples\exapp2 abc -h

Commands:
    last [Experimental] : List the last several letters in the alphabet.

Help of itself: message

> python examples\exapp2 abc last -h

Command
    exapp2 abc last : List the last several letters in the alphabet.
        This command is experimental and not covered by customer support. Please use with
        discretion.

Invocation: message

> python examples\exapp2 abc last
This command is experimental and not covered by customer support. Please use with discretion.

Argument

Help of parent command: [Tag] + message

> python examples\exapp2 ga range -h

Arguments
    --end [Experimental] : The higher bound.  Default: 5.
        Argument '--end' is experimental and not covered by customer support. Please use with
        discretion.

Invocation with this argument: message

> python examples\exapp2 ga range --end 3
Argument '--end' is experimental and not covered by customer support. Please use with discretion.

[yonzhan]

add to S167