add info on bridging two landmasses

changelog.txt

@@ -49,6 +49,9 @@ Template for new versions:
 - `gui/control-panel`: include option for turning off dumping of old clothes for `tailor`, for players who have magma pit dumps and want to save old clothes from being dumped into the magma
 - `position`: report current historical era (e.g., "Age of Myth")
 
+## Documentation
+- `gui/embark-anywhere`: add information about how the game determines world tile pathability and instructions for bridging two landmasses
+
 ## Removed
 
 # 50.13-r4

docs/gui/embark-anywhere.rst

@@ -11,8 +11,13 @@ embark in an inaccessible location on top of a mountain range? Go for it! Want
 to try a brief existence in the middle of the ocean? Nobody can stop you! Want
 to tempt fate by embarking *inside of* a necromancer tower? !!FUN!!
 
+If you are using this tool to create a fort that will bridge two disconnected
+areas of land, see `So you want to bridge a gap?`_ below for tips and caveats.
+
 Any and all consequences of embarking in strange locations are up to you to
-handle (possibly with other `armok <armok-tag-index>` tools).
+handle (possibly with other `armok <armok-tag-index>` tools). In particular,
+embarking in inaccessible locations will prevent migrants, caravans, and
+visitors from arriving.
 
 Usage
 -----
@@ -21,9 +26,37 @@ Usage
 
     gui/embark-anywhere
 
-The command will only work when you are on the screen where you can choose your
-embark site. The DFHack logo is not displayed on that screen since its default
-position conflicts with the vanilla embark size selection panel. Remember that
-you can bring up DFHack's `context menu <hotkeys>` with
-:kbd:`Ctrl`:kbd:`Shift`:kbd:`C` or the
-`in-game command launcher <gui/launcher>` directly with the backtick key (\`).
+The command will only work when you are on the screen where you can choose the
+embark site for your fort.
+
+So you want to bridge a gap?
+----------------------------
+
+A popular use case for this tool is to create a fort (or a series of forts) that
+bridges two disconnected landmasses so sites on the two landmasses can reach
+each other (that is, they can send raiding parties and/or engage in trade).
+
+However, the way this works is not entirely intuitive.
+
+A single large embark is not necessarily going to functionally connect the two
+shores so that armies can cross the gap. You could still choose to use this
+approach to build a continuous constructed bridge in fort mode for later use as
+an *adventurer* in adventure mode, but it will not be usable by the other
+characters/armies in the world.
+
+The DF world map is divided into blocks of 16x16 tiles. When you are choosing
+where to embark and you move the mouse so that your embark area "shadow" moves
+over a little bit -- that's one "tile". An embark area can span block
+boundaries, and there is no indication on the map where those boundaries are.
+
+The way DF determines world pathability is to check if the ground is continuous
+**or** if the enclosing 16x16 block contains the upper left tile of a fort
+embark area.
+
+In order for a connection to be formed for armies, one fort upper left corner
+must exist in each 16x16 block that contains part of the gap.
+
+Therefore, the simplest solution for making a "bridge" that armies can use (but
+walking adventurers cannot) is to make a 1x1 fort every 16 tiles across the
+water gap, starting on land on one shore and finishing on land on the opposite
+shore. That will ensure that every 16x16 block in the gap is covered by a fort.