Add missing docs in sov-accounts

[bkolad]

Description

This PR introduces the following changes:

1. Adds missing docs in sov-accounts module
2. Adds #![deny(missing_docs)] lint sov-accounts.

Linked Issues

• Related to Cleanup: Add missing docs/tests to existing code #438

Testing

CI passes.

Docs

Added doscs in sov-accounts module.

[codecov]

Codecov Report

Merging #545 (bfe90a9) into nightly (ea7cd9d) will decrease coverage by 0.1%.
The diff coverage is 100.0%.

Impacted Files	Coverage Δ
...em/module-implementations/sov-accounts/src/call.rs	80.7% <ø> (ø)
...m/module-implementations/sov-accounts/src/query.rs	92.8% <ø> (ø)
...tem/module-implementations/sov-accounts/src/lib.rs	95.2% <100.0%> (ø)

... and 1 file with indirect coverage changes