D28 and D27 are clamping diodes; whenever either of the data lines sinks below ground, the diode will source current into it to clamp it back to ground. The polarity of the diodes here is correct. Note that because RS-485 is designed to operate with a wide common-mode range these diodes will wrongly clamp the signal when common mode dips below ground (as pointed out by Dan Mills in a comment). As a result, your system will not operate when this happens, even though RS-485 transceivers should (by spec) be able to operate in this case. Considering that the absolute maximum rating for the bus terminals allows voltage down to -10 V, this squanders a significant portion of the allowable common-mode range. You may want to consider back-to-back Zeners instead, in order to clamp on the range of (-10 V, 15 V).
C69 is used for terminating the transmission line. This guide by TI discusses AC termination in section 4.3; the arrangement typically consists of a resistor and capacitor in series, rather than simply a capacitor. Typically the resistor is equal in value to the characteristic impedance of the transmission line. During a transition, the capacitor initially looks like a short-circuit, and the two parts together terminate the line with the resistor's impedance. During the steady-state, the capacitor blocks DC to save power and limit current draw.
The arrangement in your schematic seems odd, since the capacitor alone will present the wrong termination impedance because of the missing resistor. While you might be able to get away with it like this, you might encounter significant reflections from the receiver, especially if the transmission line is long compared to the signal wavelength.