Add documentation on signal aspects

advtrains/doc/advtrains_speed_lessp.3advtrains.md

@@ -1,4 +1,4 @@
-% advtrains.speed.lessp(3advtrains) | Advtrains Developer's Manual
+% advtrains_speed_lessp(3advtrains) | Advtrains Developer's Manual
 
 # NAME
 `advtrains.speed.lessp`, `advtrains.speed.greaterp`, `advtrains.speed.not_lessp`, `advtrains.speed_not_greaterp`, `advtrains.speed.equalp`, `advtrains.speed.not_equalp`, `advtrains.speed.max`, `advtrains.speed.min` - speed restriction comparison functions

advtrains/doc/advtrains_speed_set_restriction.3advtrains.md

@@ -1,4 +1,4 @@
-% advtrains.speed.set_restriction(3advtrains) | Advtrains Developer's Manual
+% advtrains_speed_set_restriction(3advtrains) | Advtrains Developer's Manual
 
 # NAME
 `advtrains.speed.set_restriction`, `advtrains.speed.merge_aspect` - modify speed restriction

advtrains/doc/signal_aspect.7advtrains.md

@@ -0,0 +1,24 @@
+% signal_aspect(7advtrains) | Advtrains Developer's Manual
+
+# DESCRIPTION
+
+The signal aspect table used by advtrains has the following fields:
+
+* `main`: The main speed restriction
+* `dst`: The `main` aspect of the distant signal (not implemented)
+* `type`: The type of speed restriction given by the signal
+* `shunt`: Whether shunting is allowed
+* `proceed_as_main`: Whether to proceed without shunting
+
+The `main` and `dst` fields may contain the following values:
+* `-1`: No speed restriction
+* `nil`: No information is available
+
+The `type` field can be any valid table index, but it should usually be one of the following values:
+* "main": The main signal aspect used before the introduction of speed restriction types. This is the default value if the `type` field is absent.
+* "line": The speed limit for the physical line.
+* "temp": The speed limit that is temporarily introduced.
+
+# NOTES
+
+A signal with the `main` aspect of zero should not provide distant signal aspect.