Terrain help doesn't distinguish between Cultivate and Irrigate

[lmoureaux]

Describe the bug

The two are now separate, but the help keeps merging them.

To Reproduce
Steps to reproduce the behavior:

1. Go to Help > Terrain > Swamp
2. Look at the help for Irrig. Reslt
3. Swamp can't be irrigated, only cultivated

Expected behavior

It shows "Cultivate result" or something similar.

Screenshots

Platform and version (please complete the following information):

• OS: ?, probably any
• Freeciv21 version: 3.0-alpha9
• Ruleset/Longturn game (if applicable): classic or Sim30

Additional context

freeciv21/client/gui-qt/helpdlg.cpp

Lines 1355 to 1364 in 79e53d6

	if (pterrain->irrigation_result != pterrain
	&& pterrain->irrigation_result != T_NONE
	&& pterrain->cultivate_time != 0
	&& action_id_univs_not_blocking(ACTION_CULTIVATE, NULL, &for_terr)) {
	char buffer[1024];
	fc_snprintf(buffer, sizeof(buffer),
	PL_("%d turn", "%d turns", pterrain->cultivate_time),
	pterrain->cultivate_time);
	str = N_("Irrig. Rslt/Time:");