[doc] Update build help

And update links

build/oils-preamble.sh

@@ -50,17 +50,19 @@ Compile/link flags:
   LDFLAGS=             [default ''] (defined in build/ninja-rules-cpp.sh)
     Space-separated list of more linker flags
 
-Compiler flags come from 4 sources:
+Compiler flags come from these sources:
 
   1. The $BASE_CXXFLAGS var
   2. -I $REPO_ROOT is hard-coded
   3. The build --variant, e.g. 'asan' adds -fsanitizer=address and more
-  4. The $CXXFLAGS var
+  4. Flags detected by ./configure, e.g. for GNU readline
+  5. The $CXXFLAGS var
 
-Linker flags come from 3 sources:
+Linker flags come from tehse sources:
 
   1. The build --variant, e.g. 'asan' adds -fsanitizer=address
-  2. $STRIP_FLAGS, a variable detected by ./configure
+  2. Flags detected by ./configure, like $STRIP_FLAGS and -lreadline for GNU
+     readline
   3. The $LDFLAGS var
 
 EOF

doc/index.md

@@ -30,7 +30,7 @@ See [Published Docs](published.html) for those that are ready to read.
 
 - [INSTALL](INSTALL.html). How do I install Oils?  This text file is also in
   the `oils-for-unix` tarball.
-  - [Oils Help Mirror](help-mirror.html)
+  - [Oils Build `--help` Mirror](help-mirror.html)
   - [Portability](portability.html)
 - [Getting Started](getting-started.html).  How do I use the shell?
 

doc/published.md

@@ -18,7 +18,7 @@ Build/packaging:
 
 - [INSTALL](INSTALL.html). How do I install Oils?  This text file is also in
   the `oils-for-unix` tarball.
-  - [Oils Help Mirror](help-mirror.html)
+  - [Oils Build `--help` Mirror](help-mirror.html)
   - [Portability](portability.html)
 
 [help]: ref/chap-builtin-cmd.html#help