Add missing doc for the sov-modules-stf-template crate.

[bkolad]

Description

sov-modules-stf-template crate compiles with the #![deny(missing_docs)] lint.

Linked Issues

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

Testing

No impact on tests.

Docs

Missing docs provided.

[codecov]

Codecov Report

Merging #540 (06ddb1b) into nightly (fca041c) will decrease coverage by 0.1%.
The diff coverage is 100.0%.

Impacted Files	Coverage Δ
...odule-system/sov-modules-stf-template/src/batch.rs	100.0% <ø> (+42.8%)	⬆️
...system/sov-modules-stf-template/src/tx_verifier.rs	100.0% <ø> (ø)
...ystem/sov-modules-stf-template/src/app_template.rs	82.7% <100.0%> (ø)
module-system/sov-modules-stf-template/src/lib.rs	97.4% <100.0%> (ø)

... and 14 files with indirect coverage changes

[neysofu]

Just small nitpicks:

1. The first paragraph of the doc comment is the one shown when looking at the module overview; it should be concise and it's established practice to skip the subject if necessary e.g. "This enum is...". We can be more verbose from the second paragraph onwards.
2. I think it's best if we link to all relevant Rust types and items.
3. doctest = false disables doc tests when running cargo test but not when running cargo test --doc, which is what people normally use when testing doctest changes. Instead, I'd suggest marking code examples in the README as rust,ignore and removing doctest = false.