doc: Update picolibc section in 3.5 migration guide for 1.8.4

Picolibc 1.8.4 includes more control over printf capabilities, document
those in the migration guide.

Signed-off-by: Keith Packard <keithp@keithp.com>

doc/releases/migration-guide-3.5.rst

@@ -133,6 +133,15 @@ there are a few side effects to be aware of when migrating to Picolibc.
   compiler will now warn about declarations of `main` which don't conform to
   the Zephyr required type -- ``int main(void)``.
 
+* Picolibc has three different printf/scanf variants, 'float', 'integer',
+  and 'minimal. 'float' offers a complete printf implementation with exact
+  floating point in decimal and hexidecimal formats, full integer support
+  including long long, C99 integer size specifiers (j, z, t) and POSIX
+  positional arguments. 'integer' mode removes float support, 'minimal'
+  mode also removes support for format modifiers, positional arguments and
+  long long integers. Building the library as a module allows finer
+  control over the feature set provided at each level.
+
 * Picolibc's default floating point input/output code is larger than the
   minimal C library version (this is necessary to conform with the C
   language "round trip" requirements for these operations). If you use