INAV has had power/current limiting since version 3.0.0 but it was
never documented in the main Battery.md documentation file. This
commit adds comprehensive documentation for the existing feature.

Added sections:
- Power and Current Limiting overview
- Why use power limiting (battery protection, safety, compliance)
- How it works (burst vs continuous limits, PI controller)
- Configuration settings table (current and power limits)
- Three practical example configurations
- Understanding burst mode with timeline example
- OSD elements for monitoring
- Calibration tips and best practices

The power limiting feature protects batteries and ESCs by smoothly
reducing throttle when current or power exceeds configured limits.
It uses a sophisticated burst reserve system that allows brief
high-power maneuvers while protecting during sustained flight.

Settings documented:
- limit_cont_current, limit_burst_current
- limit_burst_current_time, limit_burst_current_falldown_time
- limit_cont_power, limit_burst_power
- limit_burst_power_time, limit_burst_power_falldown_time
- limit_pi_p, limit_pi_i, limit_attn_filter_cutoff

This documentation helps users understand and configure a feature
that has existed for years but remained largely unknown due to lack
of user-facing documentation.

The power limiting initialization incorrectly treated burst limit of 0
as numerically less than continuous limit, and would automatically set
burst = continuous. However, 0 means 'disabled/unlimited' not 'zero
amps/watts allowed'.

This fix allows the valid configuration:
- limit_cont_current = 1000 (100A continuous limit)
- limit_burst_current = 0 (no burst limiting - unlimited bursts)

Changes:
- Only enforce burst >= continuous when burst is enabled (> 0)
- Applied to both current and power limit initialization
- Added comments explaining the 0 = disabled semantics

Changed divisor from 1000 to 100 to correctly convert from mAh to dA.

Example: 1500 mAh × 50C / 100 = 750 dA (75A) ✓
Previously: 1500 mAh × 50C / 1000 = 75 dA (7.5A) ✗

Thanks to Qodo code review for catching this.