fftools/ffplay: improve keyboard shortcut documentation

Clarify the behavior of seek keyboard shortcuts in both the
documentation and command-line help text. Specifically:
- left/right: mention custom interval option support
- page down/up: improve wording for chapter seeking fallback

doc/ffplay.texi

@@ -259,15 +259,13 @@ Pause if the stream is not already paused, step to the next video
 frame, and pause.
 
 @item left/right
-Seek backward/forward 10 seconds.
+Seek backward/forward by 10 seconds or a custom interval if -seek_interval is set.
 
 @item down/up
 Seek backward/forward 1 minute.
 
 @item page down/page up
-Seek to the previous/next chapter.
-or if there are no chapters
-Seek backward/forward 10 minutes.
+Seek to previous/next chapter or backward/forward 10 minutes if no chapters.
 
 @item right mouse click
 Seek to percentage in file corresponding to fraction of width.

fftools/ffplay.c

@@ -3826,9 +3826,9 @@ void show_help_default(const char *opt, const char *arg)
            "c                   cycle program\n"
            "w                   cycle video filters or show modes\n"
            "s                   activate frame-step mode\n"
-           "left/right          seek backward/forward 10 seconds or to custom interval if -seek_interval is set\n"
+           "left/right          seek backward/forward by 10 seconds or a custom interval if -seek_interval is set\n"
            "down/up             seek backward/forward 1 minute\n"
-           "page down/page up   seek backward/forward 10 minutes\n"
+           "page down/page up   seek to previous/next chapter or backward/forward 10 minutes if no chapters\n"
            "right mouse click   seek to percentage in file corresponding to fraction of width\n"
            "left double-click   toggle full screen\n"
            );