Replace bounds decoding ascii art with bytefield diagram (#526)

Replace the ascii art with the bounds decoding diagram to make it clear
how they are computed and what the resulting width of top and base.

This PR replaces this:

```
top:    t = { {1'b0, a[MXLEN - 1:E + MW]} + ct, T[MW - 1:0], {E{1'b0}} }
base:   b = {        a[MXLEN - 1:E + MW]  + cb, B[MW - 1:0], {E{1'b0}} }
```

With the bytefield figures  in the attached image.
<img width="694" alt="Screenshot 2025-02-06 at 19 21 42" src="https://github.com/user-attachments/assets/521e4239-f23d-4bd2-b9a0-e242cf35cc41" />

src/cap-description.adoc

@@ -445,12 +445,17 @@ Reconstituting the top two bits of T:
 T[MW - 1:MW - 2] = B[MW - 1:MW - 2] + LCout + LMSB
 ```
 
-Decoding the bounds:
+The bounds are decoded as shown in xref:top_bound_dec[xrefstyle=short] and
+xref:base_bound_dec[xrefstyle=short].
+
+.Decoding of the MXLEN+1 wide top (_t_) bound
+[#top_bound_dec]
+include::img/top-bound-dec.edn[]
+
+.Decoding of the MXLEN wide base (_b_) bound
+[#base_bound_dec]
+include::img/base-bound-dec.edn[]
 
-```
-top:    t = { {1'b0, a[MXLEN - 1:E + MW]} + ct, T[MW - 1:0], {E{1'b0}} }
-base:   b = {        a[MXLEN - 1:E + MW]  + cb, B[MW - 1:0], {E{1'b0}} }
-```
 The corrections c~t~ and c~b~ are calculated as as shown below using the
 definitions in xref:cap_encoding_ct[xrefstyle=short] and
 xref:cap_encoding_cb[xrefstyle=short].

src/img/base-bound-dec.edn

@@ -0,0 +1,16 @@
+[bytefield]
+----
+(defattrs :plain [:plain {:font-family "M+ 1p Fallback" :font-size 25}])
+(def row-height 80)
+(def row-header-fn nil)
+(def left-margin 100)
+(def right-margin 100)
+(def boxes-per-row 32)
+(draw-column-headers {:height 50 :font-size 22 :labels (reverse ["0" "" "" "" "" "" "" "" "" "" "E" "" "" "" "" "" "" "" "" "" "E+MW" "" "" "" "" "" "" "" "" "" "" "MXLEN-1"])})
+
+(draw-box "a[MXLEN - 1:E + MW] + cb" {:span 12})
+(draw-box "B[MW - 1:0]"              {:span 10})
+(draw-box "0"                        {:span 10})
+
+(draw-box "MXLEN" {:span 32 :borders {}})
+----

src/img/top-bound-dec.edn

@@ -0,0 +1,16 @@
+[bytefield]
+----
+(defattrs :plain [:plain {:font-family "M+ 1p Fallback" :font-size 25}])
+(def row-height 80)
+(def row-header-fn nil)
+(def left-margin 100)
+(def right-margin 100)
+(def boxes-per-row 32)
+(draw-column-headers {:height 50 :font-size 22 :labels (reverse ["0" "" "" "" "" "" "" "" "" "" "E" "" "" "" "" "" "" "" "" "" "E+MW" "" "" "" "" "" "" "" "" "" "" "MXLEN"])})
+
+(draw-box "{1'b0, a[MXLEN - 1:E + MW]} + ct" {:span 12})
+(draw-box "T[MW - 1:0]"              {:span 10})
+(draw-box "0"                        {:span 10})
+
+(draw-box "MXLEN+1" {:span 32 :borders {}})
+----