Added a note to config.md re: some values cannot be changed at run-time. (#1681)
Added a note to config.md re: some values cannot be changed at run-time. (#1681)

* Update config.md

Added a note to config.md re: some values cannot be changed at run-time. Details based on @brian-mann 's comments to https://github.com/cypress-io/cypress/issues/2100

* minor editing - fix url to urlHash

* Fix urlHash reference


Co-authored-by: Jennifer Shehane <b76474446821c6f7360e4eee990dc1f8d635e16a@gmail.com>
cypress-io/cypress-documentation,cypress-io/cypress-documentation