add documentation on equity index symbols

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.87%. Comparing base (734448a) to head (42c7d69).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #145 +/- ## ======================================= Coverage 98.87% 98.87% ======================================= Files 18 18 Lines 2217 2217 Branches 227 227 ======================================= Hits 2192 2192 Misses 17 17 Partials 8 8 ``` | [Flag](https://app.codecov.io/gh/alexgolec/schwab-py/pull/145/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alex+Golec) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/alexgolec/schwab-py/pull/145/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alex+Golec) | `98.87% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alex+Golec#carryforward-flags-in-the-pull-request-comment) to find out more.

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