Update docs to mention gitignore changes (#15134)

Co-authored-by: awsr <43862868+awsr@users.noreply.github.com>
Co-authored-by: SUZUKI Sosuke <aosukeke@gmail.com>

docs/ignore.md

@@ -24,7 +24,7 @@ coverage
 
 It’s recommended to have a `.prettierignore` in your project! This way you can run `prettier --write .` to make sure that everything is formatted (without mangling files you don’t want, or choking on generated files). And – your editor will know which files _not_ to format!
 
-By default prettier ignores files in version control systems directories (".git", ".svn" and ".hg") and `node_modules` (if [`--with-node-modules` CLI option](cli.md#--with-node-modules) not specified)
+By default prettier ignores files in version control systems directories (".git", ".sl", ".svn" and ".hg") and `node_modules` (unless the [`--with-node-modules` CLI option](cli.md#--with-node-modules) is specified). Prettier will also follow rules specified in the ".gitignore" file if it exists in the same directory from which it is run.
 
 So by default it will be
 

docs/install.md

@@ -47,7 +47,7 @@ build
 coverage
 ```
 
-> Tip! Base your .prettierignore on .gitignore and .eslintignore (if you have one).
+> Tip! Prettier will follow rules specified in .gitignore if it exists in the same directory from which it is run. You can also base your .prettierignore on .eslintignore (if you have one).
 
 > Another tip! If your project isn’t ready to format, say, HTML files yet, add `*.html`.