Some cosmetic and clarity issues with METADATA

[Herringway]

A handful of minor problems found while implementing METADATA last night:

• A few typos: "subscriping" instead of "subscribing", "responsds" instead of "responds"
• METADATA GET's syntax implies it is legal to send 0 key parameters, which is inconsistent with METADATA SUB and co.
• METADATA CLEAR's description mentions a KEY_NO_PERMISSION reply, but the standard replies table below only shows TARGET_INVALID
• TARGET_INVALID is listed in the same table as a possible response to all current subcommands, but is mentioned in only one of them
• METADATA SUBS's description mentions the response is a batch, but only one of the examples has a BATCH message in it (19 out of the 20 examples are batchless)
• METADATA SET's syntax includes a colon, implying a value with a space in it would be sent as METADATA target SET key ::value with spaces
• The "batch types" section doesn't mention that standard replies may be included in the batches