Update getAllEntries() explainer to use getAllRecords() proposal

[SteveBeckerMSFT]

Changes include:

• Rename getAllEntries() to getAllRecords()
• Replace entries results with an array of IDBRecords. Each IDBRecord contains attributes: value, key, and primaryKey.
• Move WebIDL section to the bottom.
• Add API description section before "Key Scenarios" section.
• Update JavaScript iterator example to use JavaScript generator.
• Add "Considered alternatives" section, which contains getAllEntries() as well as adding a direction option to getAll()/getAllKeys().