instrument identifier clarification for 2 char case

[crotwell]

See also https://github.com/iris-edu/mseed3-evaluation/issues/2#issuecomment-312684254

The example in the instrument identifier section gives a 2 char code that is a "sub-instrument-type" of an existing 1 char type, ie WU in LWUS is a subtype of a W instrument. We do have this:

Extended instrument codes are not necessarily limited to extending sensor families defined using a single instrument code.

but I feel this is still confusing. The primary example should be of a totally new 2 char instrument code that has no relation to a preexisting 1 char instrument code. Say a "foobarmeter" with instrument code FB. If needed, we can also say that they FDSN may wish to define some 2 char instrument codes that are sub-types of existing 1 char codes,. But this is not the default or even primary reason for the expansion.

[chad-earthscope]

OK, I'll put in something else and change the emphasis to being new instruments with only a mentioned that they may be extensions as you suggest.

As noted in the 20170718 Identifiers document:

this example should be replaced with real codes once the first is defined

but good to change the emphasis.