vcpkg_replace_string: update docs (#367)

vcpkg/maintainers/functions/vcpkg_replace_string.md

@@ -10,12 +10,13 @@ Replace a string in a file.
 ## Usage
 
 ```cmake
-vcpkg_replace_string(<filename> <match> <replace>)
+vcpkg_replace_string(<filename> <match> <replace> [REGEX] [IGNORE_UNCHANGED])
 ```
 
 ## Parameters
 
 ### `<filename>`
+
 The path to the file to modify.
 
 ### `<match>`
@@ -26,6 +27,14 @@ The string to match.
 
 The string to replace all matched sequences with.
 
+### `[REGEX]`
+
+If this argument is present, the `<match>` argument is treated as a regular expression.
+
+### `[IGNORE_UNCHANGED]`
+
+Disable the warning if the file is not changed.
+
 ## Source
 
 [scripts/cmake/vcpkg\_replace\_string.cmake](https://github.com/Microsoft/vcpkg/blob/master/scripts/cmake/vcpkg_replace_string.cmake)