docs: render the fcmp tables as code (#3735)

Looking at [the `fcmp`
documentation](https://docs.rs/cranelift-codegen/0.80.0/cranelift_codegen/ir/trait.InstBuilder.html#method.fcmp)--generated
from Cranelift's instruction definitions, the charts explaining the
logic for the various conditions is unreadable. Since rendering those charts
as plain text is problematic, this change wraps them as code sections
for a consistent layout.

cranelift/codegen/meta/src/shared/instructions.rs

@@ -3035,12 +3035,14 @@ pub(crate) fn define(
         Two IEEE 754-2008 floating point numbers, `x` and `y`, relate to each
         other in exactly one of four ways:
 
+        ```text
         == ==========================================
         UN Unordered when one or both numbers is NaN.
         EQ When `x = y`. (And `0.0 = -0.0`).
         LT When `x < y`.
         GT When `x > y`.
         == ==========================================
+        ```
 
         The 14 `floatcc` condition codes each correspond to a subset of
         the four relations, except for the empty set which would always be
@@ -3049,6 +3051,7 @@ pub(crate) fn define(
         The condition codes are divided into 7 'ordered' conditions which don't
         include UN, and 7 unordered conditions which all include UN.
 
+        ```text
         +-------+------------+---------+------------+-------------------------+
         |Ordered             |Unordered             |Condition                |
         +=======+============+=========+============+=========================+
@@ -3066,13 +3069,15 @@ pub(crate) fn define(
         +-------+------------+---------+------------+-------------------------+
         |ge     |GT | EQ     |uge      |UN | GT | EQ|Greater than or equal    |
         +-------+------------+---------+------------+-------------------------+
+        ```
 
         The standard C comparison operators, `<, <=, >, >=`, are all ordered,
         so they are false if either operand is NaN. The C equality operator,
         `==`, is ordered, and since inequality is defined as the logical
         inverse it is *unordered*. They map to the `floatcc` condition
         codes as follows:
 
+        ```text
         ==== ====== ============
         C    `Cond` Subset
         ==== ====== ============
@@ -3083,6 +3088,7 @@ pub(crate) fn define(
         `>`  gt     GT
         `>=` ge     GT | EQ
         ==== ====== ============
+        ```
 
         This subset of condition codes also corresponds to the WebAssembly
         floating point comparisons of the same name.