fix(api): disallow some more functions during textlock

Problem: nvim_buf_set_text(), nvim_open_term() and termopen() all change buffer
text, which is forbidden during textlock. Additionally, nvim_open_term() and
termopen() may be used to convert the cmdwin buffer into a terminal buffer,
which is weird.

Solution: Allow nvim_buf_set_text() and nvim_open_term() in the cmdwin, but
disallow nvim_open_term() from converting the cmdwin buffer into a terminal
buffer. termopen() is not allowed in the cmdwin (as it always operates on
curbuf), so just check text_locked().

Also happens to improve the error in #21055: nvim_buf_set_text() was callable
during textlock, but happened to check textlock indirectly via u_save();
however, this caused the error to be overwritten by an unhelpful "Failed to
save undo information" message when msg_list == NULL (e.g: an `<expr>` mapping
invoked outside of do_cmdline()).

runtime/doc/api.txt

@@ -1200,6 +1200,9 @@ nvim_open_term({buffer}, {opts})                            *nvim_open_term()*
     |nvim_chan_send()| can be called immediately to process sequences in a
     virtual terminal having the intended size.
 
+    Attributes: ~
+        not allowed when |textlock| is active
+
     Parameters: ~
       • {buffer}  the buffer to use (expected to be empty)
       • {opts}    Optional parameters.
@@ -2426,6 +2429,9 @@ nvim_buf_set_text({buffer}, {start_row}, {start_col}, {end_row}, {end_col},
     Prefer |nvim_buf_set_lines()| if you are only adding or deleting entire
     lines.
 
+    Attributes: ~
+        not allowed when |textlock| is active
+
     Parameters: ~
       • {buffer}       Buffer handle, or 0 for current buffer
       • {start_row}    First line index