Add docs for multicell + control codes

Author: kovidgoyal

docs/text-sizing-protocol.rst

@@ -262,3 +262,46 @@ There are many controls used to edit existing screen content such as
 inserting characters, deleting characters and lines, etc. These were all
 originally specified for the one character per cell paradigm. Here we specify
 their interactions with multicell characters.
+
+**Insert characters** (``CSI @`` aka ``ICH``)
+    When inserting ``n`` characters at cursor position ``x, y`` all characters
+    after ``x`` on line ``y`` are supposed to be right shifted. This means
+    that any multiline character that intersects with the cells on line ``y`` at ``x``
+    and beyond must be erased. Any single line multicell character that is
+    split by the cells at ``x`` and ``x + n - 1`` must also be erased.
+
+**Delete characters** (``CSI P`` aka ``DCH``)
+    When deleting ``n`` characters at cursor position ``x, y`` all characters
+    after ``x`` on line ``y`` are supposed to be left shifted. This means
+    that any multiline character that intersects with the cells on line ``y`` at ``x``
+    and beyond must be erased. Any single line multicell character that is
+    split by the cells at ``x`` and ``x + n - 1`` must also be erased.
+
+**Erase characters** (``CSI X`` aka ``ECH``)
+    When erasing ``n`` characters at cursor position ``x, y`` the ``n`` cells
+    starting at ``x`` are supposed ot be cleared. This means that any multicell
+    character that intersects with the ``n`` cells starting at ``x`` must be
+    erased.
+
+**Erase display** (``CSI J`` aka ``ED``)
+    Any multicell character intersecting with the erased region of the screen
+    must be erased. When using mode ``22`` the contents of the screen are first
+    copied into the scrollback, including all multicell characters.
+
+**Erase in line** (``CSI K`` aka ``EL``)
+    Works just like erase characters above. Any multicell character
+    intersecting with the erased cells in the line is erased.
+
+**Insert lines** (``CSI L`` aka ``IL``)
+    When inserting ``n`` lines at cursor position ``y`` any multiline
+    characters that are split at the line ``y`` must be erased. A split happens
+    when the second or subsequent row of the multiline character is on the line
+    ``y``. The insertion causes ``n`` lines to be removed from the bottom of
+    the screen, any multiline characters are split at the bottom of the screen
+    must be erased. A split is when any row of the multiline character except
+    the last row is on the last line of the screen after the insertion of ``n``
+    lines.
+
+**Delete lines** (``CSI M`` aka ``DL``)
+    When deleting ``n`` lines at cursor position ``y`` any multicell character
+    that intersects the deleted lines must be erased.