Documentation for Contract Module (#2045)

* Add: documentation for srml_contract, per @sergeis comments. TODO: make concise and add usage example

* Fix: minor spelling and grammar

* Add: Revisions to the doc clarification from @Sergei on trx revert behavior

* Edit: added spacing btw function list for better readability

* Remove: usage create contract example todo later

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <nicolezhuis@gmail.com>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <nicolezhuis@gmail.com>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <nicolezhuis@gmail.com>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <nicolezhuis@gmail.com>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <nicolezhuis@gmail.com>

* Edit: explained max gas limit revert behavior in greater detail

* Edit: incorporated first round of PRs, mostly syntax/styling

* Edit: incorporated Sergeis mental model, terms, and added usage examples

* Fix: staking module is no longer closely related to contract. Balances now manages the reaping of accounts

* Minor cleanup of links/text that are not ready to be shown

* Update srml/contract/src/lib.rs

made more concise

Co-Authored-By: nczhu <nicole@parity.io>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <nicole@parity.io>

* Minor edits from PR

* Explained contract creation in more detail, fixed PR suggestions

* Linted according to contri guidelines

* Created a notable scenarios section, unlisted public functions in lieu of a direct link to the module

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <nicole@parity.io>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <nicole@parity.io>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <nicole@parity.io>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <nicole@parity.io>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <nicole@parity.io>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <nicole@parity.io>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <nicole@parity.io>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <nicole@parity.io>

* Update srml/contract/src/tests.rs

Co-Authored-By: nczhu <nicole@parity.io>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <nicole@parity.io>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <nicole@parity.io>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <nicole@parity.io>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <nicole@parity.io>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <nicole@parity.io>

* minor commit after pr feedback

* s/codehash/code_hash for consistency

substrate/srml/contract/src/tests.rs

@@ -243,7 +243,7 @@ fn account_removal_removes_storage() {
 	with_externalities(
 		&mut ExtBuilder::default().existential_deposit(100).build(),
 		|| {
-			// Setup two accounts with free balance above than exsistential threshold.
+			// Set up two accounts with free balance above the existential threshold.
 			{
 				Balances::deposit_creating(&1, 110);
 				AccountInfoOf::<Test>::insert(1, &AccountInfo {
@@ -264,7 +264,7 @@ fn account_removal_removes_storage() {
 			}
 
 			// Transfer funds from account 1 of such amount that after this transfer
-			// the balance of account 1 is will be below than exsistential threshold.
+			// the balance of account 1 will be below the existential threshold.
 			//
 			// This should lead to the removal of all storage associated with this account.
 			assert_ok!(Balances::transfer(Origin::signed(1), 2, 20));