feat(docs): func level docstrings for balance module (#100)

Co-authored-by: BobTheBuidler <[email protected]>

eth_portfolio/_loaders/balances.py

@@ -14,6 +14,28 @@
 
 @stuck_coro_debugger
 async def load_token_balance(token: y.ERC20, address: Address, block: Block) -> Balance:
+    """
+    Asynchronously fetch the ERC20 token balance and its USD value for a given address at a specific block.
+
+    Args:
+        token: The ERC20 token contract to query.
+        address: The address holding the ERC20 tokens.
+        block: The block number for the balance query.
+
+    Returns:
+        :class:`~eth_portfolio.typing.Balance`: A custom object containing:
+            - balance: The token balance (in token's smallest unit).
+            - value: The USD value of the balance (18 decimal places).
+            - token: The ERC20 token which was checked.
+            - block: The block number where the balance was taken.
+
+    Note:
+        Non-standard ERC20 tokens are handled gracefully, returning a zero balance.
+
+    Example:
+        >>> balance = await load_token_balance(token=dai_contract, address='0x1234...', block=12345678)
+        >>> print(f"Token: {balance.token}, Value: {balance.balance}, USD: {balance.usd_value}")
+    """
     try:
         balance = await token.balance_of_readable(address, block, sync=False)
     except y.NonStandardERC20:
@@ -28,6 +50,24 @@ async def load_token_balance(token: y.ERC20, address: Address, block: Block) ->
 
 
 def _calc_value(balance, price) -> Decimal:
+    """
+    Calculate the USD value of a token balance based on its price.
+
+    Args:
+        balance: The token balance.
+        price: The token price in USD.
+
+    Returns:
+        The total USD value, rounded to 18 decimal places if possible.
+        If rounding is not possible due to high precision, returns the unrounded value.
+
+    Note:
+        Returns :class:`~decimal.Decimal(0)` if the price is None, handling cases where price data is unavailable.
+
+    Example:
+        >>> value = _calc_value(balance=1000, price=0.50)
+        >>> print(f"USD Value: {value}")
+    """
     if price is None:
         return Decimal(0)
     # NOTE If balance * price returns a Decimal with precision < 18, rounding is both impossible and unnecessary.