Misc spec clarifications

There are a few clarifications that could improve the spec (without changing any content). This could be done for a version 0.4.1 or just rolled into 0.5.

• Specify json encoding https://github.com/ome/ngff/issues/81
• Constrain dtypes & shape for multiscale arrays: https://github.com/ome/ngff/issues/2
• Clarify purpose of multiple multiscales https://github.com/ome/ngff/issues/17
• Clarify json terminology https://github.com/ome/ngff/issues/66
• Text vs. markdown quotes: https://github.com/ome/ngff/issues/38
• Single version information at top level, see https://github.com/ome/ngff/issues/83. (That would be a change of the metadata, but only superficial)
• '.' as separator disallowed before 0.5: https://github.com/ome/ngff/issues/135
• whether or not multiple images are supported before 0.5: #112
• interpretation of the multiscales in a single image group

Review the linked issues for the JSON, dtype and shape, multiscale, versioning, separator, and image-support questions, then locate the current NGFF specification text they concern. Define the clarification scope and whether each item belongs in a release before editing; done means the specification resolves all listed questions without unintended content changes.