Using indexers: specify attribute to use for the custom name

[pkulikov]

Main change is to fix the following line (line 31) by specifying the attribute that should be used:

To provide the indexer with a name that other languages can use, use a name attribute in the declaration. For example:

Other changes:

• add language identifiers
• use backticks where appropriate
• remove mention of the DateTime property from the first paragraph as it doesn't help the topic
• remove h3 headers from the example sections.

[BillWagner]

Thanks for making these changes @pkulikov

I've reviewed the changes, and I'll now.

Thanks again for submitting to docs.