Add assert reference documentation

Perhaps this doc needs to be improved with more concrete / useful examples. For now, this will have to do.

Author: aedart

docs/+current/modules/testing/cmake/-asserts.md

@@ -7,4 +7,210 @@ author: RSP Systems A/S
 
 # Asserts
 
-_TODO: ..._
+[TOC]
+
+## Failure Message
+
+All assert functions support an optional `MESSAGE` argument, which is shown if the assertion failed.
+
+```cmake
+assert_truthy(false MESSAGE "My fail msg...")
+```
+
+## Existence
+
+### `assert_defined()`
+
+Assert key to be defined.
+
+```cmake
+assert_defined(my_variable)
+```
+
+See https://cmake.org/cmake/help/latest/command/if.html#defined.
+
+### `assert_not_defined()`
+
+The opposite of `assert_defined()`. 
+
+```cmake
+assert_not_defined(my_variable)
+```
+
+## Boolean
+
+### `assert_truthy()`
+
+Assert key to be truthy.
+
+```cmake
+assert_truthy(my_variable)
+```
+
+See https://cmake.org/cmake/help/latest/command/if.html#basic-expressions.
+
+### `assert_falsy()`
+
+The opposite of `assert_truthy()`.
+
+```cmake
+assert_falsy(my_variable)
+```
+
+## Numbers
+
+### `assert_equals()`
+
+Assert numeric keys or values equal each other..
+
+```cmake
+assert_equals(expected actual)
+```
+
+See https://cmake.org/cmake/help/latest/command/if.html#equal.
+
+### `assert_not_equals()`
+
+The opposite of `assert_equals()`.
+
+```cmake
+assert_not_equals(expected actual)
+```
+
+### `assert_less_than()`
+
+Assert numeric key or value is less than specified value.
+
+```cmake
+assert_less_than(expected actual)
+```
+
+See https://cmake.org/cmake/help/latest/command/if.html#less.
+
+### `assert_less_than_or_equal()`
+
+Assert numeric key or value is less than or equal to the specified value.
+
+```cmake
+assert_less_than_or_equal(expected actual)
+```
+
+See https://cmake.org/cmake/help/latest/command/if.html#less-equal.
+
+### `assert_greater_than()`
+
+Assert numeric key or value is greater than specified value.
+
+```cmake
+assert_greater_than(expected actual)
+```
+
+See https://cmake.org/cmake/help/latest/command/if.html#greater.
+
+### `assert_greater_than_or_equal()`
+
+Assert numeric key or value is greater than or equal to the specified value.
+
+```cmake
+assert_greater_than_or_equal(expected actual)
+```
+
+See https://cmake.org/cmake/help/latest/command/if.html#greater-equal.
+
+## Strings
+
+### `assert_string_equals()`
+
+Assert string keys or values equal each other.
+
+```cmake
+assert_string_equals(expected actual)
+```
+
+See https://cmake.org/cmake/help/latest/command/if.html#strequal
+
+### `assert_string_not_equals()`
+
+Opposite of `assert_string_equals()`.
+
+```cmake
+assert_string_not_equals(expected actual)
+```
+
+### `assert_string_empty()`
+
+Assert given string is empty.
+
+```cmake
+assert_string_empty("${my_string}")
+```
+
+See https://cmake.org/cmake/help/latest/command/string.html#length
+
+### `assert_string_not_empty()`
+
+Opposite of `assert_string_empty()`.
+
+```cmake
+assert_string_not_empty("${my_string}")
+```
+
+## Lists
+
+### `assert_in_list()`
+
+Assert key (value) to be in given list.
+
+```cmake
+assert_in_list(item list)
+```
+
+See https://cmake.org/cmake/help/latest/command/if.html#in-list.
+
+### `assert_not_in_list()`
+
+Opposite of `assert_in_list()`.
+
+```cmake
+assert_not_in_list(item list)
+```
+
+## Commands & Macros
+
+### `assert_is_callable()`
+
+Assert key to be a callable command or macro.
+
+```cmake
+assert_is_callable("my_function")
+```
+
+See https://cmake.org/cmake/help/latest/command/if.html#command.
+
+### `assert_is_not_callable()`
+
+Opposite of `assert_is_callable()`.
+
+```cmake
+assert_is_not_callable("my_unknown_function")
+```
+
+## Files & Paths
+
+### `assert_file_exists()`
+
+Assert file exists.
+
+```cmake
+assert_file_exists("${my_file_path}")
+```
+
+See https://cmake.org/cmake/help/latest/command/if.html#exists.
+
+### `assert_file_not_exists()`
+
+Opposite of `assert_file_exists()`.
+
+```cmake
+assert_file_not_exists("${my_file_path}")
+```