Relax frequency requirements in us_ticker.md

Apply changes from PR #1123 to v5.13.

Author: Amanda Butler

docs/porting/target/us_ticker.md

@@ -6,7 +6,8 @@ Implementing the microsecond ticker enables Mbed OS to perform operations that r
 
 ### Defined behavior
 
-- Has a reported frequency between 250KHz and 8MHz.
+- Has a reported frequency between 250 KHz and 8 MHz for counters that are less than 32 bits wide.
+- Has a reported frequency up to 100 MHz for counters that are 32 bits wide.
 - Has a counter that is at least 16 bits wide.
 - The function `ticker_init` is safe to call repeatedly.
 - The function `ticker_init` allows the ticker to keep counting and disables the ticker interrupt.
@@ -17,7 +18,7 @@ Implementing the microsecond ticker enables Mbed OS to perform operations that r
 - The ticker interrupt fires only when the ticker time increments to or past the value set by `ticker_set_interrupt`.
 - It is safe to call `ticker_set_interrupt` repeatedly before the handler is called.
 - The function `ticker_fire_interrupt` causes `ticker_irq_handler` to be called immediately from interrupt context.
-- The ticker operations `ticker_read`, `ticker_clear_interrupt`, `ticker_set_interrupt` and `ticker_fire_interrupt` take less than 20us to complete.
+- The ticker operations `ticker_read`, `ticker_clear_interrupt`, `ticker_set_interrupt` and `ticker_fire_interrupt` take less than 20 us to complete.
 - The ticker operations `ticker_init` and `ticker_read` are atomic.
 
 ### Undefined behavior
@@ -31,13 +32,13 @@ Implementing the microsecond ticker enables Mbed OS to perform operations that r
 
 Be careful around these common trouble areas when implementing this API:
 
-- The ticker cannot drift when rescheduled repeatedly
-- The ticker keeps counting when it rolls over
-- The ticker interrupt fires when the compare value is set to 0 and overflow occurs
+- The ticker cannot drift when rescheduled repeatedly.
+- The ticker keeps counting when it rolls over.
+- The ticker interrupt fires when the compare value is set to 0 and overflow occurs.
 
 ## Dependencies
 
-To implement this API, the device must have a hardware counter that has a count value at least 16 bits wide and can operate between 250KHz and 8MHz.
+To implement this API, the device must have a hardware counter that has a count value at least 16 bits wide and can operate between 250 KHz and 8 MHz.
 
 ## Implementing the microsecond ticker API