abcoathup GSN Bouncers review (#1916)

* Fix typo

* Replace pseudo code contracts with sample code

* Update GSN Bouncers text

* More text changes

* Update with latest code and remove reference to allowance

* Capitalize Custom Bouncer

* Update docs/modules/ROOT/pages/gsn-bouncers.adoc

Co-Authored-By: Francisco Giordano <frangio.1@gmail.com>

* Update gsn-bouncers.adoc with Antora cross reference

Co-Authored-By: Francisco Giordano <frangio.1@gmail.com>

* Revert to handling msg.sender msg.data differently

* Change by default to simplest implementation

* Change signing to include signature for GSNBouncerSignature

* Reword summary of what is in the guide

* Remove "The" from before `GSNBouncer...`

* Fix code snippet markdown

* Change to API references to xref:api

* Remove code from How it works sections

* Explain 1:1 exchange rate

* Change transaction request to relayed call

* Minor fixes

* Add info to Custom Bouncers

* Typo

* Minor fixes

* reorder sentence based on review gsn-bouncers.adoc

Co-Authored-By: Francisco Giordano <frangio.1@gmail.com>

* Improve wording of signing of relayed call parameters by trusted signer