M7: say what a default font family in files does and doesn't promise

Review: a saved family renders only where that family is available. The
docs and the test's comment now say the variable changes only what new
text starts in, and that a file keeps its fonts in a build with another
default, falling back like any missing family elsewhere.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/doc/src/text.rs

@@ -797,7 +797,7 @@ mod case_tests {
         assert_eq!(DEFAULT_FONT, named.unwrap_or("Inter"));
         let st = TextStyle::default();
         assert_eq!(st.font_family, DEFAULT_FONT);
-        // The family is always written, so a saved file opens in the same font in any build.
+        // The family is always written, so a saved file keeps it in a build with another default.
         let saved = serde_json::to_value(&st).unwrap();
         assert_eq!(saved["font_family"], DEFAULT_FONT);
     }

docs/releasing.md

@@ -79,8 +79,10 @@ other fonts) or one its users have installed:
 FIGCRAFT_DEFAULT_FONT="Noto Sans Thai" cargo build --release -p figcraft
 ```
 
-Files record each text's family, so they open in the same font in any build. The release workflow
-leaves it unset.
+Files record each text's family, so the variable only changes what new text starts in: a file
+saved by one build keeps its fonts in a build with another default. Like any other family, it
+renders only where it is available (embedded or installed); elsewhere the text falls back to the
+faces that are. The release workflow leaves it unset.
 
 ## What gets built