docs(contracts): document loan manager lifecycle methods

[gitcommit90]

Closes #1133

Add focused /// rustdoc blocks to the six public loan_manager lifecycle functions that currently have no documentation: request_loan, approve_loan, repay, deposit_collateral, liquidate, and cancel_loan.

Each doc block covers:

• Required authorization (borrower, admin, or liquidator)
• Pause/status preconditions (LoanStatus variants required)
• Return behavior and side-effects (fund transfers, balance updates, TTL bumps)
• All returned LoanError variants with the condition that triggers each

No runtime behavior changes — rustdoc comments only.

Verification:

• rustfmt --check --config skip_children=true loan_manager/src/lib.rs — exit 0
• cargo check -p loan_manager --lib — passes
• RUSTDOCFLAGS=-Dwarnings cargo doc -p loan_manager --no-deps — passes, no warnings
• cargo clippy -p loan_manager --lib -- -D warnings — passes
• cargo test -p loan_manager — pre-existing compile error in untouched test.rs (unrelated soroban_sdk::Val PartialEq issue); this branch touches only lib.rs
• git diff main...HEAD --name-only — contracts/loan_manager/src/lib.rs only