Document BLEStringCharacteristic in API reference

[rupayon123]

Problem

The API reference lists the typed characteristic constructors but omits BLEStringCharacteristic, leaving its constructor and String methods undocumented (#349).

Change

Add it to the constructor list and document its maximum byte size, writeValue / setValue behavior, value() return type, and a small local-characteristic example. I checked the descriptions against src/BLEStringCharacteristic.h, src/BLEStringCharacteristic.cpp, and the underlying local write path.

Validation

• git diff --check passed.
• Compared each documented signature and value-size behavior with the current source.
• Documentation-only change; no device or example compilation was run locally (arduino-cli is unavailable).

Prepared with AI assistance. Fixes #349.

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