docs: clarify the semantics of the required: kwarg everywhere

Link to feature options consistently, and point out that it controls
"whether" the function finds what it's trying to find. This clues people
in to the fact that disabled features exist.

docs/yaml/functions/dependency.yaml

@@ -158,9 +158,13 @@ kwargs:
     type: bool | feature
     default: true
     description: |
-      when set to false, Meson will proceed with the build
-      even if the dependency is not found. *(since 0.47.0)* The value of a
-      [`feature`](Build-options.md#features) option can also be passed.
+      When set to `false`, Meson will proceed with the build
+      even if the dependency is not found.
+
+      When set to a [`feature`](Build-options.md#features) option, the feature
+      will control if it is searched and whether to fail if not found.
+
+      *(since 0.47.0)* The value of a `feature` option can also be passed.
 
   static:
     type: bool

docs/yaml/functions/import.yaml

@@ -19,8 +19,14 @@ posargs:
 kwargs:
   required:
     type: bool | feature
+    default: true
     since: 0.59.0
-    description: Whether the mdule is required and Meson should abort if not found.
+    description:
+      When set to `false`, Meson will proceed with the build even if the module
+      is not found.
+
+      When set to a [`feature`](Build-options.md#features) option, the feature
+      will control if it is searched and whether to fail if not found.
 
   disabler:
     type: bool

docs/yaml/objects/compiler.yaml

@@ -137,7 +137,11 @@ methods:
       type: bool | feature
       default: false
       since: 0.50.0
-      description: If set to `true`, Meson will halt if the header check fails.
+      description:
+        When set to `true`, Meson will halt if the header check fails.
+
+        When set to a [`feature`](Build-options.md#features) option, the feature
+        will control if it is searched and whether to fail if not found.
 
 
 # Star of the actual functions
@@ -367,8 +371,10 @@ methods:
         be found. Otherwise, Meson will continue and the found method of the
         returned object will return `false`.
 
-        *(since 0.47.0)* The value of a [`feature`](Build-options.md#features)
-        option can also be passed here.
+        When set to a [`feature`](Build-options.md#features) option, the feature
+        will control if it is searched and whether to fail if not found.
+
+        *(since 0.47.0)* The value of a `feature` option can also be passed here.
 
     has_headers:
       type: list[str]
@@ -609,4 +615,3 @@ methods:
       type: list[str]
       description: |
         Extra flags to pass to the preprocessor
-