Document/complete feature flags more

doc_src/fish.txt

@@ -29,4 +29,6 @@ The following options are available:
 
 - `-D` or `--debug-stack-frames=DEBUG_LEVEL` specify how many stack frames to display when debug messages are written. The default is zero. A value of 3 or 4 is usually sufficient to gain insight into how a given debug call was reached but you can specify a value up to 128.
 
+- `-f` or `--features=FEATURES` enables one or more feature flags (separated by a comma). These are how fish stages changes that might break scripts.
+
 The fish exit status is generally the exit status of the last foreground command. If fish is exiting because of a parse error, the exit status is 127.

doc_src/status.txt

@@ -16,6 +16,8 @@ status function
 status line-number
 status stack-trace
 status job-control CONTROL-TYPE
+status features
+status test-feature FEATURE
 \endfish
 
 \subsection status-description Description
@@ -47,9 +49,13 @@ The following operations (sub-commands) are available:
 
 - `line-number` prints the line number of the currently running script. Also `current-line-number`, `-n` or `--current-line-number`.
 
+- `stack-trace` prints a stack trace of all function calls on the call stack. Also `print-stack-trace`, `-t` or `--print-stack-trace`.
+
 - `job-control CONTROL-TYPE` sets the job control type, which can be `none`, `full`, or `interactive`. Also `-j CONTROL-TYPE` or `--job-control=CONTROL-TYPE`.
 
-- `stack-trace` prints a stack trace of all function calls on the call stack. Also `print-stack-trace`, `-t` or `--print-stack-trace`.
+- `features` lists all available feature flags.
+
+- `test-feature FEATURE` returns 0 when FEATURE is enabled, 1 if it is disabled, and 2 if it is not recognized.
 
 \subsection status-notes Notes