manpage: document the dangers of some subtitle options

Setting some subtitle options may lead to incorrect rendering of complex
ASS subtitle scripts, such as displaced signs or visual artifacts. The
user should be made aware that this can happen.

In theory, libass could make using some of these options relatively
safe, but it doesn't.

Note that there are potentially much more options that could in theory
break subtitle rendering, but add a warning only to the most fragile
ones.

DOCS/man/en/options.rst

@@ -109,14 +109,17 @@
     - ``--aspect=16:9`` or ``--aspect=1.7777``
 
 --ass, --no-ass
-    Use libass to render all text subtitles. This enables support for the
-    native styling of SSA/ASS subtitles, and also support for some styling
-    features in other subtitle formats by conversion to ASS markup. Enabled by
-    default if the player was compiled with libass support.
+    Render ASS subtitles natively, and convert text subtitles in other formats
+    to ASS internally (enabled by default).
 
-    *NOTE*: With ``--ass``, some of the ASS subtitle options work for non-ASS
-    text subtitles only, because ASS subtitles include their own styling
-    information.
+    If ``--no-ass`` is specified, all subtitles are converted to plain text
+    internally. All tags and style declarations are stripped and ignored. The
+    subtitle renderer uses the font style as specified by the ``--sub-text-``
+    options instead.
+
+    *NOTE*: Using ``--no-ass`` may lead to incorrect or completely broken
+    rendering of ASS/SSA subtitles. It can sometimes be useful to forcibly
+    override the styling of ASS subtitles, but should be avoided in general.
 
 --ass-force-style=<[Style.]Param=Value[,...]>
     Override some style or script info parameters.
@@ -126,6 +129,8 @@
     - ``--ass-force-style=FontName=Arial,Default.Bold=1``
     - ``--ass-force-style=PlayResY=768``
 
+    *NOTE*: Using this option may lead to incorrect subtitle rendering.
+
 --ass-hinting=<type>
     Set hinting type. <type> can be:
 
@@ -144,6 +149,8 @@
     rendering text subtitles. The syntax of the file is exactly like the ``[V4
     Styles]`` / ``[V4+ Styles]`` section of SSA/ASS.
 
+    *NOTE*: Using this option may lead to incorrect subtitle rendering.
+
 --ass-style-override=<yes|no>
     Control whether user style overrides should be applied.
 
@@ -1937,9 +1944,15 @@
     Specify the position of subtitles on the screen. The value is the vertical
     position of the subtitle in % of the screen height.
 
+    *NOTE*: this affects ASS subtitles as well, and may lead to incorrect
+    subtitle rendering. Use with care, or use ``--sub-text-margin-y`` instead.
+
 --sub-scale=<0-100>
     Factor for the text subtitle font size (default: 1).
 
+    *NOTE*: this affects ASS subtitles as well, and may lead to incorrect
+    subtitle rendering. Use with care, or use ``--sub-text-font-size`` instead.
+
 --sws=<n>
     Specify the software scaler algorithm to be used with ``--vf=scale``. This
     also affects video output drivers which lack hardware acceleration,