Align text to what is allowed in ISO 19115/19139

[PeterParslow]

The first sentence of the current wording says explicitly that you can have more than one (lower in the hierarchy) gmd:extent within the gmd:EX_TemporalExtent. Neither 19115 nor 19139 allow this.

The last sentence of the current wording appears to say that if you want more than one date/date period, e.g. for a discontinuous temporal extent, then you should create more than one gmd:EX_Extent within your gmd:extent - but again neither 19115 nor 19139 allows this.

The allowable way to have "more than one extent" (either temporal or spatial) is to have more than one gmd:extent within the gmd:MD_DataIdentification. Each TemporalExtent can only have one gmd:extent, and each of those can only contain one gml:TimePeriod (or other TM_Primitive)