ztest: Add docstring for zassume macro

Add a detailed docstring to the zassume() macro that describes what it does
and how its usage is different from zassert.

Signed-off-by: Aaron Massey <aaronmassey@google.com>

subsys/testsuite/ztest/include/zephyr/ztest_assert.h

@@ -141,6 +141,24 @@ static inline bool z_zassume(bool cond, const char *default_msg, const char *fil
 		}                                                                                  \
 	} while (0)
 
+/**
+ * @brief Skip the test, if @a cond is false
+ *
+ * You probably don't need to call this macro directly. You should
+ * instead use zassume_{condition} macros below.
+ *
+ * Note that when CONFIG_MULTITHREADING=n macro returns from the function. It's then expected that
+ * in that case ztest assumes will be used only in the context of the test function.
+ *
+ * NOTE: zassume should not be used to replace zassert, the goal of zassume is to skip tests that
+ * would otherwise fail due to a zassert on some other dependent behavior that is *not* under test,
+ * thus reducing what could be tens to hundreds of assertion failures to investigate down to a few
+ * failures only.
+ *
+ * @param cond Condition to check
+ * @param msg Optional, can be NULL. Message to print if @a cond is false.
+ * @param default_msg Message to print if @a cond is false
+ */
 #define zassume(cond, default_msg, msg, ...)                                                       \
 	do {                                                                                       \
 		bool _ret = z_zassume(cond, msg ? ("(" default_msg ")") : (default_msg), __FILE__, \