Add documentation on signal aspects

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+% advtrains_speed_set_restriction(3advtrains) | Advtrains Developer's Manual
+
+# NAME
+`advtrains.speed.set_restriction`, `advtrains.speed.merge_aspect` - modify speed restriction
+
+# SYNOPSIS
+* `advtrains.speed.set_restriction(train, rtype, rval)`
+* `advtrains.speed.merge_aspect(train, asp)`
+
+# DESCRIPTION
+
+The `advtrains.speed.set_restriction` function sets the speed restriction of type `rtype` of `train` to `rval` and updates the speed restriction value to the strictest speed restriction in the table, or `nil` if all speed restrictions are `nil` or `-1`. If the speed restriction table does not exist, it is created with the `"main"` speed restriction being the speed restriction value of `train`.
+
+The `advtrains.speed.merge_aspect` function merges the main aspect of `asp` into the speed restriction table with the same procedure described above. If the signal aspect table does not provide the type of speed restriction, the restriction type `"main"` is assumed.
+
+# SIDE EFFECTS
+
+Both functions modify `train.speed_restriction` and `train.speed_restrictions_t`.