docs: enable syntax highlighting in several docs files

... for better readability

Closes #6286

docs/DYNBUF.md

@@ -11,7 +11,9 @@ without using the dedicated dynbuf API.
 
 ## init
 
-    void Curl_dyn_init(struct dynbuf *s, size_t toobig);
+```c
+void Curl_dyn_init(struct dynbuf *s, size_t toobig);
+```
 
 This inits a struct to use for dynbuf and it can't fail. The `toobig` value
 **must** be set to the maximum size we allow this buffer instance to grow to.
@@ -19,44 +21,58 @@ The functions below will return `CURLE_OUT_OF_MEMORY` when hitting this limit.
 
 ## free
 
-    void Curl_dyn_free(struct dynbuf *s);
+```c
+void Curl_dyn_free(struct dynbuf *s);
+```
 
 Free the associated memory and clean up. After a free, the `dynbuf` struct can
 be re-used to start appending new data to.
 
 ## addn
 
-    CURLcode Curl_dyn_addn(struct dynbuf *s, const void *mem, size_t len);
+```c
+CURLcode Curl_dyn_addn(struct dynbuf *s, const void *mem, size_t len);
+```
 
 Append arbitrary data of a given length to the end of the buffer.
 
 ## add
 
-    CURLcode Curl_dyn_add(struct dynbuf *s, const char *str);
+```c
+CURLcode Curl_dyn_add(struct dynbuf *s, const char *str);
+```
 
 Append a C string to the end of the buffer.
 
 ## addf
 
-    CURLcode Curl_dyn_addf(struct dynbuf *s, const char *fmt, ...);
+```c
+CURLcode Curl_dyn_addf(struct dynbuf *s, const char *fmt, ...);
+```
 
 Append a `printf()`-style string to the end of the buffer.
 
 ## vaddf
 
-    CURLcode Curl_dyn_vaddf(struct dynbuf *s, const char *fmt, va_list ap);
+```c
+CURLcode Curl_dyn_vaddf(struct dynbuf *s, const char *fmt, va_list ap);
+```
 
 Append a `vprintf()`-style string to the end of the buffer.
 
 ## reset
 
-    void Curl_dyn_reset(struct dynbuf *s);
+```c
+void Curl_dyn_reset(struct dynbuf *s);
+```
 
 Reset the buffer length, but leave the allocation.
 
 ## tail
 
-    CURLcode Curl_dyn_tail(struct dynbuf *s, size_t length)
+```c
+CURLcode Curl_dyn_tail(struct dynbuf *s, size_t length);
+```
 
 Keep `length` bytes of the buffer tail (the last `length` bytes of the
 buffer). The rest of the buffer is dropped. The specified `length` must not be
@@ -64,7 +80,9 @@ larger than the buffer length.
 
 ## ptr
 
-    char *Curl_dyn_ptr(const struct dynbuf *s);
+```c
+char *Curl_dyn_ptr(const struct dynbuf *s);
+```
 
 Returns a `char *` to the buffer if it has a length, otherwise a NULL. Since
 the buffer may be reallocated, this pointer should not be trusted or used
@@ -72,7 +90,9 @@ anymore after the next buffer manipulation call.
 
 ## uptr
 
-    unsigned char *Curl_dyn_uptr(const struct dynbuf *s);
+```c
+unsigned char *Curl_dyn_uptr(const struct dynbuf *s);
+```
 
 Returns an `unsigned char *` to the buffer if it has a length, otherwise a
 NULL. Since the buffer may be reallocated, this pointer should not be trusted
@@ -80,7 +100,9 @@ or used anymore after the next buffer manipulation call.
 
 ## len
 
-    size_t Curl_dyn_len(const struct dynbuf *s);
+```c
+size_t Curl_dyn_len(const struct dynbuf *s);
+```
 
 Returns the length of the buffer in bytes. Does not include the terminating
 zero byte.