Add information about 3.3v current ability and UART rx_buffer_size

[codewise-nicolas]

Added two paragraphs in the uart pinout section about 3.3v current sourcing on the bms, and about uart buffer size problems and fix.

Description:

Related issue (if applicable): fixes

Pull request in esphome with YAML changes (if applicable): esphome/esphome#<esphome PR number goes here>

Checklist:

• [x] Branch: next is for changes and new documentation that will go public with the next ESPHome release. Fixes, changes and adjustments for the current release should be created against current.
• [x] Link added in /index.rst when creating new documents for new components or cookbook.

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[codewise-nicolas]

If this addition is not wanted, decline my PR!

[nagyrobi]

@codewise-nicolas patience...

[codewise-nicolas]

@jesserockz thanks for the suggestions, ive accepted/commited them to the pr.