Expressions: a mixed number such as 2 1/8" is 2.125" (#1802) (#1820)

Thanks @SychPL.

docs/book/src/guide/parameters.md

@@ -19,7 +19,9 @@ Expressions are unit-aware: `2 * width + 5 mm`, `angle / 2`, `sqrt(area)`, `1 in
 `+ - * / ^`, parentheses, `sin cos tan asin acos atan atan2 sqrt abs min max floor ceil round mod
 trunc` and unit conversion. A decimal comma works too, outside parentheses: `2,5 mm` is
 `2.5 mm` (inside a function's parentheses the comma separates its arguments, so write
-`max(2.5, 3)` there). A length where an angle is expected (or the other way round) is an
+`max(2.5, 3)` there). A mixed number is one value, as imperial sizes are written: `2 1/8"` and
+`3 7/16 in` are 2.125 in and 3.4375 in (a whole number, a space, then a proper fraction of whole
+numbers). A length where an angle is expected (or the other way round) is an
 error, not a silent conversion. Cycles are reported with their path (`a → b → a`), renaming a
 parameter updates every expression, dimension and feature input that uses it, and a parameter
 in use can't be deleted (the error lists its users).